b43/legacy: fix beacon change processing

Process beacon change even if the BSSID doesn't
change at the same time. Also fix what I think
is a small locking error in b43legacy, there's
a spin_unlock_irqrestore that looks out of place.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>