V4L/DVB: cx25840/ivtv: replace ugly priv control with s_config

The cx25840 used a private control CX25840_CID_ENABLE_PVR150_WORKAROUND
to be told whether to enable a workaround for certain pvr150 cards.

This is really config data that it needs to get at load time.

Implemented this in cx25840 and ivtv.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>