net: dsa: mv88e6xxx: call vtu_getnext directly in db load/purge

mv88e6xxx_vtu_getnext is simple enough to call it directly in the
mv88e6xxx_port_db_load_purge function and explicit the return code
expected by switchdev for software VLANs when an hardware VLAN does
not exist.

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