cfg80211: introduce cfg80211_get_chan_state

Helper function for finding out which channel is
used by a given interface.

An exclusive channel can be used only by a single
interface. This is mainly for non-fixed channel
IBSS handling.

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