mac80211: Disallow to set multicast BSSID

Okay, here is the first of the five patches. After applying all
of them you should be able to build/join huge city mesh networks
(e.g. with the OLSR protocol) with the most of the mac80211 wireless
drivers by setting a fixed BSSID in the ad hoc mode. (If you found no
other bug/problem.) This was not specified in the original standard,
but is a widely used de facto standard.

The first patch now completely disallow to set multicast MAC addresses
as BSSID. The behavior before was really strange.

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