cfg80211: fix locking in action frame TX

Accesses to "wdev->current_bss" must be
locked with the wdev lock, which action
frame transmission is missing.

Cc: stable@kernel.org [2.6.33+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>