cfg80211: check iface combinations only when iface is running

commit f8cdddb8d61d16a156229f0910f7ecfc7a82c003 upstream.

Don't validate interface combinations on a stopped
interface. Otherwise we might end up being able to
create a new interface with a certain type, but
won't be able to change an existing interface
into that type.

This also skips some other functions when
interface is stopped and changing interface type.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>