ssb: implement spurious tone avoidance

And make use of it in b43. This fixes a regression introduced with
49d55cef


b43: N-PHY: implement spurious tone avoidance
This commit made BCM4322 use only MCS 0 on channel 13, which of course
resulted in performance drop (down to 0.7Mb/s).

Reported-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>