net: dsa: mv88e6xxx: move generic VTU GetNext

Even though every switch model has a different way to access the VTU
Data bits, the base implementation of the VTU GetNext operation remains
the same: wait, write the first VID to iterate from, start the
operation, and read the next VID.

Move this generic implementation into global1_vtu.c and abstract the
handling of the start VID (similarly to the ATU GetNext implementation),
before introducing a new chip operation for specific chips.

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