Add tracking for self-generated frames when the VAP is in sleep state.

The hardware can generate its own frames (eg RTS/CTS exchanges, other
kinds of 802.11 management stuff, especially when it comes to 802.11n)
and these also have PWRMGT flags.  So if the VAP is asleep but the
NIC is in force-awake for some reason, ensure that the self-generated
frames have PWRMGT set to 1.

Now, this (like basically everything to do with powersave) is still
racy - the only way to guarantee that it's all actually consistent
is to pause transmit and let it finish before transitioning the VAP
to sleep, but this at least gets the basic method of tracking and
updating the state debugged.

Tested:

* AR5416, STA mode
* AR9380, STA mode
Ignore the error from pipespace_new when creating a pipe.

It can fail if pipe map is exhausted (as a result of too many pipes created),
but it is not fatal and could be provoked by unprivileged users. The only
consequence is worse performance with given pipe.

Reported by:	ivoras
Suggested by:	kib
MFC after:	1 week
Switch to use arm_devmap_add_entry() to setup static device mapping.

Approved by:	stas (mentor)