cfg80211: protect first access of last_request on 11d hint under mutex

We were not protecting last_request there is a small possible race
between an 11d hint and another routine which calls reset_regdomains()
which can prevent a valid country IE from being processed. This is
not critical as it will still be procesed soon after but locking prior
to it is correct.

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