[media] tvaudio: don't report mono when stereo is received

The V4L2 spec says reporting mono and stereo at the same time means
the hardware can not distinguish between the two. So when we can,
we should report only one of them.

Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>