net/mlx5e: Fix HW MTU settings

Previously we configured HW MTU to be netdev->mtu, actually we
need to configure netdev->mtu + (ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN).

Also, query MTU can not fail, hence make the relevant helper a
void functionm, add mlx5e_set_dev_port_mtu, helper function to
handle MTU setting.

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