V4L/DVB (12612): si4713: simplify the code to remove a compiler warning.

The compiler warned about an uninitialized stereo variable. By simplifying
the code it 1) improved readability and 2) fixed the compiler warning.

Acked-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>