net/mlx4_en: Check device state when setting coalescing

When the device is down, CQs are freed. We must check the device state
to avoid issuing firmware commands on non existing CQs.

CC: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>