cfg80211: add channel checking for iface combinations

.connect cannot be handled since the driver scans
and connects on its own. It is up to the driver
then to refuse a connection (with -EBUSY for
example).

Non-fixed channel IBSSes always take a single
channel resource. For example two non-fixed
channel IBSSes always take up 2
num_different_channels, even if they operate on
the same channel at a given point of time.

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