net/mlx5e: Tx, Unconstify SQ stop room

Use an SQ field for stop_room, and use the larger value only if TLS
is supported.

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