[media] em28xx-cards: fix a warning

drivers/media/usb/em28xx/em28xx-cards.c:3309:2: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/em28xx/em28xx-cards.c

@@ -3306,7 +3306,7 @@ static int em28xx_usb_probe(struct usb_interface *interface,
 
 	dev->num_alt = interface->num_altsetting;
 
-	if ((card[nr] >= 0) && (card[nr] < em28xx_bcount))
+	if ((unsigned)card[nr] < em28xx_bcount)
 		dev->model = card[nr];
 
 	/* save our data pointer in this interface device */