Bring over the initial static bluetooth coexistence configuration
for the WB195 combo NIC - an AR9285 w/ an AR3011 USB bluetooth NIC.

The AR3011 is wired up using a 3-wire coexistence scheme to the AR9285.

The code in if_ath_btcoex.c sets up the initial hardware mapping
and coexistence configuration.  There's nothing special about it -
it's static; it doesn't try to configure bluetooth / MAC traffic priorities
or try to figure out what's actually going on.  It's enough to stop basic
bluetooth traffic from causing traffic stalls and diassociation from
the wireless network.

To use this code, you must have the above NIC.  No, it won't work
for the AR9287+AR3012, nor the AR9485, AR9462 or AR955x combo cards.

Then you set a kernel hint before boot or before kldload, where 'X'
is the unit number of your AR9285 NIC:

# kenv hint.ath.X.btcoex_profile=wb195

This will then appear in your boot messages:

[100482] athX: Enabling WB195 BTCOEX

This code is going to evolve pretty quickly (well, depending upon my
spare time) so don't assume the btcoex API is going to stay stable.

In order to use the bluetooth side, you must also load in firmware using
ath3kfw and the binary firmware file (ath3k-1.fw in my case.)

Tested:

* AR9280, no interference
* WB195 - AR9285 + AR3011 combo; STA mode; basic bluetooth inquiries
  were enough to cause traffic stalls and disassociations.  This has
  stopped with the btcoex profile code.

TODO:

* Importantly - the AR9285 needs ASPM disabled if bluetooth coexistence
  is enabled.  No, I don't know why.  It's likely some kind of bug to do
  with the AR3011 sending bluetooth coexistence signals whilst the device
  is asleep.  Since we don't actually sleep the MAC just yet, it shouldn't
  be a problem.  That said, to be totally correct:

  + ASPM should be disabled - upon attach and wakeup
  + The PCIe powersave HAL code should never be called

  Look at what the ath9k driver does for inspiration.

* Add WB197 (AR9287+AR3012) support
* Add support for the AR9485, which is another combo like the AR9285
* The later NICs have a different signaling mechanism between the MAC
  and the bluetooth device; I haven't even begun to experiment with
  making that HAL code work.  But it should be a lot more automatic.

* The hardware can do much more interesting traffic weighting with
  bluetooth and wifi traffic.  None of this is currently used.
  Ideally someone would code up something to watch the bluetooth traffic
  GPIO (via an interrupt) and then watch it go high/low; then figure out
  what the bluetooth traffic is and adjust things appropriately.

* If I get the time I may add in some code to at least track this stuff
  and expose statistics.  But it's up to someone else to experiment with
  the bluetooth coexistence support and add the interesting stuff (like
  "real" detection of bulk, audio, etc bluetooth traffic patterns and
  change wifi parameters appropriately - eg, maximum aggregate length,
  transmit power, using quiet time to control TX duty cycle, etc.)
Don't hold the node lock over the iterator.

The "find node" function call will increase the node reference anyway;
so there's no reason to hold the node table lock during the MLME change.

The only reason I could think of is to stop overlapping mlme ioctls
from causing issues, but this should be fixed a different way.

This fixes a whole class of LORs that creep up when nodes are being
timed out or removed by hostapd.

Tested:

* AR5416, hostap, with nodes coming and going.  No LORs or stability
  issues were observed.