net: dsa: mv88e6xxx: add VTU GetNext operation

Add a new vtu_getnext operation to the chip info structure to differ the
various implementations of the VTU accesses.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>