net/mlx5e: Fix selftest for small MTUs

[ Upstream commit 228c4cd04dfd0667eda182c91504b83c17d97584 ]

Loopback test had fixed packet size, which can be bigger than configured
MTU. Shorten the loopback packet size to be bigger than minimal MTU
allowed by the device. Text field removed from struct 'mlx5ehdr'
as redundant to allow send small packets as minimal allowed MTU.

Fixes: d605d668 ("net/mlx5e: Add support for ethtool self diagnostics test")
Signed-off-by: Valentine Fatiev <valentinef@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>