V4L/DVB (5093): Pvrusb2: Emit VIDIOC_S_TUNER correctly

Audio mode changes are not private to the audio chip - other I2C
modules need to see this as well.  And since the command in question
is VIDIOC_S_TUNER which is a standard v4l2 command, we really should
be broadcasting it out.  This change sets up a broadcast pathway for
VIDIOC_S_TUNER and also eliminates the now redundant code from the
audio chip handler.
This fix enables stereo reception for the FM radio

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