mac80211: Fixed BSSID handling revisited

This patch cleanup the fixed BSSID handling, that
ieee80211_sta_set_bssid() works like ieee80211_sta_set_ssid(). So
that the BSSID is only a second selection criterion besides the
SSID. This allows us to create new IBSS networks with fixed BSSIDs,
which was broken before.

In the second version of this patch the handling of the stupid merges
to the same BSSID is moved out to get reworked into an other patch.

And this version hopefully solves the problems with some low-level
drivers and re-adds the config BSSID warning to help debugging the
low-level drivers.

Much thanks to all who have helped testing! :)

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