Correct handling of off-channel frames:
o do not override the home channel recorded for the sta when the frame is
received off-channel; this fixes a problem where we might think the sta was operating on the channel the frame was received on causing association requests to be ignored/rejected (likely cause of kern/99036)
o don't include rssi of off-channel frames in the avg rssi used to select
a bss; this gives us a better estimate of the signal we will see for the station when on-channel
PR: kern/99036
Found by: Yubin Gong
Reviewed by: sephe
MFC after: 1 week