net: dsa: mv88e6xxx: call vtu_getnext directly in vlan_add

Wrapping mv88e6xxx_vtu_getnext makes the code less easy to read and
_mv88e6xxx_port_vlan_add is the only function requiring the preparation
of a new VLAN entry.

To simplify things up, remove the mv88e6xxx_vtu_get wrapper and
explicit the VLAN lookup in _mv88e6xxx_port_vlan_add. This rework
also avoids programming the broadcast entries again when changing a
port's membership, e.g. from tagged to untagged.

At the same time, rename the helper using an old underscore convention.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>