cfg80211: disallow beacon_int_min_gcd with IBSS

This can't really be supported right now, because the IBSS
interface may change its beacon interval at any time due to
joining another network; thus, there's already "support"
for different beacon intervals here, implicitly.

Until we figure out how we should handle this case (continue
to allow it to arbitrarily join? Join only if compatible?)
disallow advertising that different beacon intervals are
supported if IBSS is allowed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>