[media] tm6000: Fix resource freeing in 'tm6000_prepare_isoc()'

'usb_free_urb(urb)' is a no-op, because urb is known to be NULL.

It is likelly that releasing resources allocated by
'tm6000_alloc_urb_buffers()' just a few lines above is expected here.

This has been spotted by the following coccinelle script:
@@
expression ret, x, e;
identifier f;
@@

*   if (x == NULL)
    {
     ... when != x = e;
(
*    f(<+...x...+>);
|
*    ret = f(<+...x...+>);
)
     ...
    }

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/tm6000/tm6000-video.c

@@ -631,7 +631,7 @@ static int tm6000_prepare_isoc(struct tm6000_core *dev)
 		urb = usb_alloc_urb(max_packets, GFP_KERNEL);
 		if (!urb) {
 			tm6000_uninit_isoc(dev);
-			usb_free_urb(urb);
+			tm6000_free_urb_buffers(dev);
 			return -ENOMEM;
 		}
 		dev->isoc_ctl.urb[i] = urb;