net/mlx4_en: Change min MTU size to ETH_MIN_MTU

NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in
the RFC791 and in the network stack. Remove old mlx4_en only define for
it, which was set to wrong value.

Fixes: b80f71f5 ("ethernet/mellanox: use core min/max MTU checking")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>