regulatory: remove locking from wiphy_apply_custom_regulatory

wiphy_apply_custom_regulatory() doesn't have to hold
the regulatory mutex as it only modifies the given
wiphy with the given regulatory domain, it doesn't
access any global regulatory data.

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