[media] cx231xx: fix handling cx231xx_read_i2c_data result

The function can return negative value.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107Signed-off-by: NAndrzej Hajda <a.hajda@samsung.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/usb/cx231xx/cx231xx-video.c

@@ -1114,7 +1114,8 @@ int cx231xx_enum_input(struct file *file, void *priv,
 	struct cx231xx_fh *fh = priv;
 	struct cx231xx *dev = fh->dev;
 	u32 gen_stat;
-	unsigned int ret, n;
+	unsigned int n;
+	int ret;
 
 	n = i->index;
 	if (n >= MAX_CX231XX_INPUT)