cfg80211: ignore consecutive equal regulatory hints

We ignore regulatory hints for the same alpha2 if we already
have processed the same alpha2 on the current regulatory domain.
For a driver regulatory_hint() this means we copy onto its
wiphy->regd the previously procesed regulatory domain from CRDA
without having to call CRDA again.

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