[media] cx231xx: enable the analog tuner at buffer setup

buf_prepare callback is called for every queued buffer. This is
an overkill. Call it at buf_setup, as this should be enough.

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