[media] au0828: only create V4L2 graph if V4L2 is registered

It doesn't make sense to try to create the analog TV graph,
if the device fails to register at V4L2, or if it doesn't have
V4L2 support.

Thanks to Shuah for pointing this issue.

Reported-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>