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

commit 5078b2e3 upstream.

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.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>