V4L/DVB (5034): Pvrusb2: Enable radio mode round #1

V4L/DVB (5034): Pvrusb2: Enable radio mode round #1

This is the logic that supports switching modes via e.g.,
  echo radio > /sys/class/pvrusb2/sn-*/ctl_input/cur_val.

To do the mode switching we need to:
a) broadcast AUDC_SET_RADIO and
b) issue the CX2341X_ENC_MUTE_VIDEO command to the encoder.

The first is done by adding a new pvr2_i2c_op and having it trigger on
input change, the second by adding this command in pvr2_encoder_start()
and requesting an encoder restart on input change by setting
stale_subsys_mask appropriately.

The clues about AUDC_SET_RADIO and CX2341X_ENC_MUTE_VIDEO were kindly
provided by Hans Verkuil on the pvrusb2 mailing list. The idea to
implement mode switching this way (on input change) is due to Mike Isely.

Why AUDC_SET_RADIO/VIDIOC_S_STD are used for switching? I can 't be sure,
but I think this can be traced to a cornell student being the first to
implement radio support in ivtv "as a different standard". I think the
rest just evolved from there (it 's in the ivtv ML archives).

Signed-off-by: Pantelis Koukousoulas <pakt223@freemail.gr>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>