net/mlx5e: Fix selftest for small MTUs

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>