cfg80211: fix race on init and driver registration

There is a theoretical race that if hit will trigger
a crash. The race is between when we issue the first
regulatory hint, regulatory_hint_core(), gets processed
by the workqueue and between when the first device
gets registered to the wireless core. This is not easy
to reproduce but it was easy to do so through the
regulatory simulator I have been working on. This
is a port of the fix I implemented there [1].

[1] https://github.com/mcgrof/regsim/commit/a246ccf81f059cb662eee288aa13100f631e4cc8

Cc: stable@vger.kernel.org
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>