net: dsa: mv88e6xxx: prepare mv88e6xxx_g1_atu_op() for the mv88e6250

All the currently supported chips have .num_databases either 256 or
4096, so this patch does not change behaviour for any of those. The
mv88e6250, however, has .num_databases == 64, and it does not put the
upper two bits in ATU control 13:12, but rather in ATU Operation
9:8. So change the logic to prepare for supporting mv88e6250.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>