cfg80211: add wiphy_apply_custom_regulatory()

This adds wiphy_apply_custom_regulatory() to be used by drivers
prior to wiphy registration to apply a custom regulatory domain.
This can be used by drivers that do not have a direct 1-1 mapping
between a regulatory domain and a country.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>