net: dsa: mv88e6xxx: extract single VLAN retrieval

Rename _mv88e6xxx_vlan_init in _mv88e6xxx_vtu_new, eventually called
from a new _mv88e6xxx_vtu_get function, which abstracts the VTU GetNext
VID-1 trick to retrieve a single entry.

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