net: dsa: mv88e6xxx: explicit entry passed to vtu_getnext

mv88e6xxx_vtu_getnext interprets two members from the input
mv88e6xxx_vtu_entry structure: the (excluded) vid member to start
the iteration from, and the valid argument specifying whether the VID
must be written or not (only required once at the start of a loop).

Explicit the assignation of these two fields right before calling
mv88e6xxx_vtu_getnext, as it is done in the mv88e6xxx_vtu_get wrapper.

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