cfg80211: fix in nl80211_set_reg()

There is a race on access to last_request and its alpha2
through reg_is_valid_request() and us possibly processing
first another regulatory request on another CPU. We avoid
this improbably race by locking with the cfg80211_mutex as
we should have done in the first place. While at it add
the assert on locking on reg_is_valid_request().

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