mac80211: Don't scan if BSSID and channel are set manually

If you set a fixed BSSID and channel it's not necessary to scan for
neighbors to merge, because you really don't want to merge with it. So
don't do it.

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