mlx4: mlx4_en_low_latency_recv() called with BH disabled

mlx4_en_low_latency_recv() is called with BH disabled,
as other ndo_busy_poll() methods.

No need for spin_lock_bh()/spin_unlock_bh()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>