net: dsa: mv88e6xxx: Fix deadlock by double lock

ethtool -S on a DSA interface can deadlock for some switches because
the same lock is taken twice. Use the register read function which
expects the lock to be already held.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Fixes: 31888234 ("net: dsa: mv88e6xxx: Replace stats mutex with SMI mutex")
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx.c

@@ -703,8 +703,8 @@ static void _mv88e6xxx_get_ethtool_stats(struct dsa_switch *ds,
 		u32 high = 0;
 
 		if (s->reg >= 0x100) {
-			ret = mv88e6xxx_reg_read(ds, REG_PORT(port),
-						 s->reg - 0x100);
+			ret = _mv88e6xxx_reg_read(ds, REG_PORT(port),
+						  s->reg - 0x100);
 			if (ret < 0)
 				goto error;
 			low = ret;