net: dsa: mv88e6xxx: call vtu_getnext directly in vlan_del

Wrapping mv88e6xxx_vtu_getnext makes the code less easy to read.
Explicit the call to mv88e6xxx_vtu_getnext in _mv88e6xxx_port_vlan_del
and the return value expected by switchdev in case of software VLANs.

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>