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: e0c69ca7dfbb ("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>
diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 8c92895..2f993e6 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c
@@ -2769,6 +2769,8 @@
 		goto unlock;
 	}
 
+	occupancy &= MV88E6XXX_G2_ATU_STATS_MASK;
+
 unlock:
 	mv88e6xxx_reg_unlock(chip);