dsa: mv88e6xxx: fix debug print for SPEED_UNFORCED

Debug print uses invalid check to detect if speed is unforced:
(speed != SPEED_UNFORCED) should be used instead of (!speed).

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: NAndrey Eremeev <Axtone4all@yandex.ru>
Fixes: 96a2b40c ("net: dsa: mv88e6xxx: add port's MAC speed setter")
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/port.c

@@ -283,7 +283,7 @@ static int mv88e6xxx_port_set_speed_duplex(struct mv88e6xxx_chip *chip,
 	if (err)
 		return err;
 
-	if (speed)
+	if (speed != SPEED_UNFORCED)
 		dev_dbg(chip->dev, "p%d: Speed set to %d Mbps\n", port, speed);
 	else
 		dev_dbg(chip->dev, "p%d: Speed unforced\n", port);
@@ -516,7 +516,7 @@ int mv88e6393x_port_set_speed_duplex(struct mv88e6xxx_chip *chip, int port,
 	if (err)
 		return err;
 
-	if (speed)
+	if (speed != SPEED_UNFORCED)
 		dev_dbg(chip->dev, "p%d: Speed set to %d Mbps\n", port, speed);
 	else
 		dev_dbg(chip->dev, "p%d: Speed unforced\n", port);