[media] v4l2: standardize log start/end message

For drivers that properly use the v4l2 framework (i.e. set v4l2_dev in the
video_device struct), the start and end messages of VIDIOC_LOG_STATUS are
now generated automatically. People tended to forget these, but the v4l2-ctl
tool scans for these messages, and it also makes it easier to read the status
output in the kernel log.

The cx18, ivtv and bttv drivers were changed since they no longer need to
log these start/end messages.

In saa7164 two empty log_status functions were removed.

Also added a helper function to v4l2-ctrl.c that can be used as the
vidioc_log_status callback if all you need to do is to log the current control
values. This is now used by pwc and vivi.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Steven Toth <stoth@kernellabs.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>