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>