[media] au0828: Refactoring for start_urb_transfer()

This issue was detected by using the Coccinelle software.

1. Let us return directly if a buffer allocation failed.

2. Delete the jump label "err" then.

3. Drop the explicit initialisation for the variable "ret"
   at the beginning.

4. Return zero as a constant at the end.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>