cfg80211: move mutex locking to reg_process_pending_hints()

This will be required in the next patch and it makes the
next patch easier to review.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: Mark Mentovai <mark@moxienet.com>
Tested-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>