b43: HT-PHY: implement spurious tone avoidance

On N-PHY it's also done after TX power fix, so it was easy to spot.
Unfortunately the MMIO logs I have from ndsiwrapper include channels
1-12 only, so enabling code for 13 and 14 is just a N-PHY-based guess.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>