net: dsa: mv88e6xxx: Fix unused variable warning by using variable

_mv88e6xxx_stats_wait() did not check the return value from
 mv88e6xxx_g1_read(), so the compiler complained about set but unused
 err.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -787,6 +787,9 @@ static int _mv88e6xxx_stats_wait(struct mv88e6xxx_chip *chip)
 
 	for (i = 0; i < 10; i++) {
 		err = mv88e6xxx_g1_read(chip, GLOBAL_STATS_OP, &val);
+		if (err)
+			return err;
+
 		if ((val & GLOBAL_STATS_OP_BUSY) == 0)
 			return 0;
 	}