mac80211/mesh: make the preq queue lock consistent

Make mesh_preq_queue_lock locking consistent with mesh_queue_preq() using
spin_lock_bh().

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>