mac80211: verify IBSS in interface combinations

Drivers shouldn't attempt to advertise support
for more than one IBSS interface since mac80211
doesn't support that. Check and return an error
from ieee80211_register_hw() in that case.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>