cfg80211: separate intersection section in __set_regdom()

So far the __set_regdom() code is pretty generic as the
intersection case is fairly straight forward; this will however
change when 802.11d support is added so lets separate intersection
code for now in preparation for 802.11d support.

This patch only has slight functional changes.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>