net/mlx4_en: Fix selftest for small MTUs

Set the minimal MTU threshold for running loopback selftest.
MTU should be big enough to include packet payload, NET_IP_ALIGN,
Ethernet headers and preamble length.

Fixes: e7c1c2c4

 ("mlx4_en: Added self diagnostics test implementation")
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>