cfg80211: force WIPHY_FLAG_CUSTOM_REGULATORY on wiphy_apply_custom_regulatory()

wiphy_apply_custom_regulatory() implies WIPHY_FLAG_CUSTOM_REGULATORY
but we never enforced it, do that now and warn if the driver
didn't set it. All drivers should be following this today already.

Having WIPHY_FLAG_CUSTOM_REGULATORY does not however mean you will
use wiphy_apply_custom_regulatory() though, you may have your own
_orig value set up tools / helpers. The intel drivers are examples
of this type of driver.

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