net/mlx5: support default RSS key as null

net/mlx5: support default RSS key as null

Applications which add RSS rules must supply an RSS key and length.
If an application is only interested in default RSS operation it
should not care about the exact RSS key.
By setting the key to NULL - the PMD will use the default RSS key.
In addition if the application does not care about the RSS type it can
set it to 0 and the PMD will use the default type (ETH_RSS_IP).

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>