cfg80211: implement multi-vif interface combination validation

This refactors cfg80211_can_use_iftype_chan() so
that it can process multi-interface combination
changes.

With this it will be possible to handle, e.g.
multi-BSS channel switching.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
(cherry picked from http://mid.gmane.org/1390226968-18928-3-git-send-email-michal.kazior@tieto.com)