net/mlx5e: Return error if prio is specified when offloading eswitch vlan push

This isn't supported when we emulate eswitch vlan push action which
is the current state of things.

Fixes: 8b32580d

 ('net/mlx5e: Add TC vlan action for SRIOV offloads')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>