net/mlx4_en: Block insertion of ethtool steering rules while the interface is down

Attaching steering rules while the interface is down is an invalid operation, block it.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>