[media] au0828: Cache the decoder info at au0828 dev structure

Instead of seeking for the decoder every time analog stream is
started, cache it. This simplifies the code a little bit.

Requested-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>