cfg80211: check radar interface combinations

To ease further DFS development regarding interface combinations, use
the interface combinations structure to test for radar capabilities.
Drivers can specify which channel widths they support, and in which
modes. Right now only a single AP interface is allowed, but as the
DFS code evolves other combinations can be enabled.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>