cfg80211/mac80211: move combination check to mac80211 for ibss

Now that mac80211 can check the interface combinations itself, move
the combinations check from cfg80211 to mac80211 when joining an IBSS.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>