media: stkwebcam: Bugfix for wrong return values

usb_control_msg returns in case of a successfully sent message the number
of sent bytes as a positive number. Don't use this value as a return value
for stk_camera_read_reg, as a non-zero return value is used as an error
condition in some cases when stk_camera_read_reg is called.
Signed-off-by: Andreas Pape <ap@ca-pape.de>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/stkwebcam/stk-webcam.c

@@ -171,7 +171,11 @@ int stk_camera_read_reg(struct stk_camera *dev, u16 index, u8 *value)
 		*value = *buf;
 
 	kfree(buf);
-	return ret;
+
+	if (ret < 0)
+		return ret;
+	else
+		return 0;
 }
 
 static int stk_start_stream(struct stk_camera *dev)