Begin fleshing out some code to handle tracking PLCP error rates
in preparation for the scan based retune logic.

The linux iwlwifi driver does a rescan (onto a non-active channel)
to force an RF retune when the PLCP error rates exceed a certain threshold.

* Add code to track HT PLCP rate errors;
* Separate out the PLCP error count fetch and update so the delta
  can be used when checking for PLCP error rates;
* Implement the PLCP error logic from iwlwifi;
* For now, just print out whenever the error rate exceeds the
  threshold.

The actual scan based retune will take a bit more effort; the scan
command code right now assumes that a scan state is passed in.
This does need to change to be more flexible (both for this and
in preparation for scanning multiple channels at once.)

Tested:

* 5100 (STA mode)
* 2200 (STA mode)
* 2230 (STA mode)
Refactor out the scan channel to be assigned early on in iwn_scan()
rather than it all being a mess of 'c' and 'ic->ic_curchan'.

Tested:

* Intel 5100 (STA)
Add a channel parameter to iwn_scan().

This is in preparation for being able to use iwn_scan() to do an off
channel scan to reset the RF tuning.

It should be a no-op.

Tested:

* Intel 5100 (STA)