net/mlx5e: Allow reporting of checksum unnecessary

Currently we practically never report checksum unnecessary, because
for all IP packets we take the checksum complete path.

Enable non-default runs with reprorting checksum unnecessary, using
an ethtool private flag. This can be useful for performance evals
and other explorations.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>