Use spin_[un]lock_bh in mwl8k_sta_notify().  The sta_notify handler is
required to be atomic, yet it can be called in process context, so make
sure one call won't preempt another.
Signed-off-by: NPavel Roskin <proski@gnu.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>