V4L/DVB (13396): correct initialization of audio_mode

This initialization of the value of audio_mode is the one used if nothing
matches in the subsequent switch.  The variable audio_mode is subsequently
assigned to constants such as TUNER_AUDIO_MONO and TUNER_AUDIO_STEREO.
TUNER_AUDIO_STEREO has the same value as V4L2_TUNER_MODE_STEREO, so it
would seem better to use that value here.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>