V4L/DVB: gspca - sonixj: Do the audio input work for webcams with a microphone

The bit 0x04 of the bridge register 02 (GPIO) is used for audio connection
in webcams containing the bridge SN9C105. This patch sets it correctly,
according to the presence of an audio device.

Tested-by: Kyle Baker <kyleabaker@gmail.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>