cfg80211: add missing device list locking

When calling into the wext code from the NETDEV_UP
notifier, we need to hold the devlist_mtx mutex as
the wext code ends up calling into channel checks.

Reported-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>