cfg80211: allow drivers to iterate over matching combinations

The patch splits cfg80211_check_combinations()
into an iterator function and a simple iteration
user.

This makes it possible for drivers to asses how
many channels can use given iftype setup. This in
turn can be used for future
multi-interface/multi-channel channel switching.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>