mac80211: Don't merge if BSSID is set manually

If you set a fixed BSSID manually, you never want that the driver
change it back, or your ad-hoc mesh network will break into peaces. So
don't do it.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>