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: NEugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: NTariq Toukan <tariqt@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>