V4L/DVB (9217): si470x: correction of mono/stereo handling

This patch corrects the behavior of mono/stereo indication and
selection.

These functions now work conform to what's defined in the V4L2
specification.

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>