net/mlx5e: Set port MTU on netdev creation rather on open

Port mtu shouldn't be written to hardware on every single interface
open.
Here we set it only when needed, on change_mtu and netdevice creation.

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