net/mlx5: E-Switch: Introduce prio tag mode

Current ConnectX HW is unable to perform VLAN pop in TX path and VLAN
push on RX path. To workaround that limitation untagged packets will be
tagged with VLAN ID 0x000 (priority tag) and pop/push actions will be
replaced by VLAN re-write actions (which are supported by the HW).
Introduce prio tag mode as a pre-step to controlling the workaround
behavior.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>