net: dsa: mv88e6xxx: support 256 databases

The 6185 family of devices has only 256 address databases. Their 8-bit
FID for ATU and VTU operations are split into ATU Control and ATU/VTU
Operation registers.

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