net: dsa: Add more convenient functions for installing port VLANs

This hides the need to perform a two-phase transaction and construct a
switchdev_obj_port_vlan struct.

Call graph (including a function that will be introduced in a follow-up
patch) looks like this now (same for the *_vlan_del function):

dsa_slave_vlan_rx_add_vid   dsa_port_setup_8021q_tagging
            |                        |
            |                        |
            |          +-------------+
            |          |
            v          v
           dsa_port_vid_add      dsa_slave_port_obj_add
                  |                         |
                  +-------+         +-------+
                          |         |
                          v         v
                       dsa_port_vlan_add

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>