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: Andrey Eremeev <[e-mail]>
Fixes: 96a2b40c7bd3 ("net: dsa: mv88e6xxx: add port's MAC speed setter")
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/mv88e6xxx/port.c

···
 	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);
···
 	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);