cfg80211: add reg_get_dfs_region()

This can be used outside of the regulatory context for any checks
on the DFS region. The central cfg80211 dfs_region is always used
and if it does not match with the wiphy a debug print is issued.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>