cfg80211: refactor cfg80211_can_use_iftype_chan()

Separate the code that counts the interface types and channels from
the code that check the interface combinations.  The new function that
checks for combinations is exported so it can be called by the
drivers.

This is done in preparation for moving the interface combinations
checks out of cfg80211.

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