V4L/DVB (11396): cx88: avoid reprogramming every audio register on A2 stereo/mono change

This patch changes cx88_set_stereo to avoid resetting all of the audio
registers on stereo/mono change if the audio standard is A2, and set
only the AUD_CTL register. The benefit of this method is that it
eliminates the annoying clicking noise on setting the audio mode to
stereo or mono.

The driver had used the same method 1.5 years ago (and for FM radio it
still does), but a pretty big cleanup commit changed it to the
"complete audio reset" method, although the reason for this move was
not clear. (If somebody knows why it was necessary, please let me
know!)

The original commit: http://linuxtv.org/hg/v4l-dvb/rev/ffe313541d7d



Signed-off-by: Marton Balint <cus@fazekas.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>