net: dsa: mv88e6xxx: write VID outside of VTU Get Next code

There is no need to write the VLAN ID before every Get Next operation,
since the VTU VID register is not cleared between calls.

Move the VID write call in a _mv88e6xxx_vtu_vid_write function outside
of _mv88e6xxx_vtu_getnext so future code could call VTU Get Next
multiple times and save a few register accesses.

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