IB/mlx4: Fix uninitialized spinlock for 32-bit archs

uar_lock spinlock was used in mlx4_ib_cq_arm without being initialized
(this only affects 32-bit archs, because uar_lock is not used on
64-bit archs and MLX4_INIT_DOORBELL_LOCK() is a NOP).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>