net/mlx5e: Support configurable MTU for vport representors

The representor MTU was hard coded to 1500 bytes.
Allow setting arbitrary MTU values up to the max supported by the FW.

Signed-off-by: Adi Nissim <adin@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>