net: dsa: mv88e6xxx: abstract VTU/STU data access

Both VTU and STU operations use the same routine to access their
(common) data registers, with a different offset.

Add VTU and STU specific read and write functions to the data registers
to abstract the required offset.

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>