net/mlx5e: Different SQ types

Different SQ types (tx, xdp, ico) are growing apart, we separate them
and remove unwanted parts in each one of them, to simplify data path and
utilize data cache.

Remove DB union from SQ structures since it is not needed anymore as we
now have different SQ data type for each SQ.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>