net/mlx5e: Don't modify CQ before it was created

Calling mlx5e_set_coalesce while the interface is down will result in
modifying CQs that don't exist.

Fixes: f62b8bb8

 ('net/mlx5: Extend mlx5_core to support ConnectX-4
Ethernet functionality')
Signed-off-by: Gal Pressman <galp@mellanox.com>

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>