cfg80211: validate IBSS BSSID

The IBSS BSSID is never validated, so an
invalid one might end up being used. Fix
this by rejecting invalid configuration.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>