cfg80211: Add regulatory domain intersection capability

There are certain scenerios where we require intersecting
two regulatory domains. This adds intersection support.
When we enable 802.11d support we will use this to intersect
the regulatory domain from the AP's country IE and what our
regulatory agent believes is correct for a country.

This patch enables intersection for now in the case where
the last regdomain was set by a country IE which was parsed
and the user then wants to set the regulatory domain. Since
we don't support country IE parsing yet this code path will not
be hit, however this allows us to pave the way for 11d support.

Intersection code has been tested in userspace with CRDA.

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