net/mlx5: remove self-assignment on esw->dev

There is a self assignment of esw->dev to itself, clean this up by
removing it. Also make dev a const pointer.

Addresses-Coverity: ("Self assignment")
Fixes: 6cedde45 ("net/mlx5: E-Switch, Verify support QoS element type")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>