[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>