V4L/DVB: tvp7002: fix query_dv_preset

If no signal or valid format is detected, then VIDIOC_QUERY_DV_PRESET must
return 0 and set the preset to V4L2_DV_INVALID.

This driver returned an error instead.

Also simplified the kernel messages a bit: change v4l2_err to v4l2_dbg (not
detecting a signal/valid format is not an error, so only print this info when
debug == 1) and do not call v4l_fill_dv_preset_info: it's overkill to do this
just for a debug message.

Thanks to Mats Randgaard for bringing this to my attention.

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