net: dsa: mv88e6xxx: Add missing mask of ATU occupancy register

Only the bottom 12 bits contain the ATU bin occupancy statistics. The
upper bits need masking off.

Fixes: e0c69ca7 ("net: dsa: mv88e6xxx: Add ATU occupancy via devlink resources")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>