cfg80211/mac80211: move more combination checks to mac80211

Get rid of the cfg80211_can_add_interface() and
cfg80211_can_change_interface() functions by moving that functionality
to mac80211.  With this patch all interface combination checks are now
out of cfg80211 (except for the channel switch case which will be
addressed in a future commit).

Additionally, modify the ieee80211_check_combinations() function so
that an undefined chandef can be passed, in order to use it before a
channel is defined.

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