cfg80211: fix race between core hint and driver's custom apply

Its possible for cfg80211 to have scheduled the work and for
the global workqueue to not have kicked in prior to a cfg80211
driver's regulatory hint or wiphy_apply_custom_regulatory().

Although this is very unlikely its possible and should fix
this race. When this race would happen you are expected to have
hit a null pointer dereference panic.

Cc: stable@kernel.org
Signed-off-by: NLuis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: NAlan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/wireless/reg.c

@@ -1551,6 +1551,13 @@ static int regulatory_hint_core(const char *alpha2)
 
 	queue_regulatory_request(request);
 
+	/*
+	 * This ensures last_request is populated once modules
+	 * come swinging in and calling regulatory hints and
+	 * wiphy_apply_custom_regulatory().
+	 */
+	flush_scheduled_work();
+
 	return 0;
 }