V4L/DVB: cx25840, v4l2-subdev, ivtv, pvrusb2: Fix ivtv/cx25840 tinny audio

This change attempts to fix the ivtv tinny audio problem by keeping digitizer
to encoder audio clocks running, while disabling the video clocks as needed to
avoid unpredictable PCI bus hangs.

To accomplish this, for the cx25840 module enabling of audio streaming had
to be separated from enabling video streaming, requiring an additional
v4l2_subdev_audio_op and calls to this new op in the pvrusb2 and ivtv drivers.

The cx231xx and cx23885 driver use the cx25840 module for affecting only
video on s_stream calls, so those drivers needed no change.

The CX23418 hardware does not exhibit either the tinny audio problem nor the PCI
bus hang, so the cx18 driver did not need corresponding changes.

CX2341[56] based cards that are not using the CX2584x family of chips
do not seem to be affected by the tinny audio problem, and this change should
not affect how they are configured. It will delay their first capture by
starting by another 300 msec though.

Many thanks go to Argus <pthorn-ivtvd@styx2002.no-ip.org> and
Martin Dauskardt <martin.dauskardt@gmx.de> whose persistent testing and
investigation of this problem will hopefully fix this problem once and for all
for many ivtv users.

Reported-by: Martin Dauskardt <martin.dauskardt@gmx.de>
Reported-by: Argus <pthorn-ivtvd@styx2002.no-ip.org>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>