net/mlx5: Declare more strictly devlink encap mode

Devlink has UAPI declaration for encap mode, so there is no
need to be loose on the data get/set by drivers.

Update call sites to use enum devlink_eswitch_encap_mode
instead of plain u8.

Suggested-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>