V4L/DVB (13070): radio-mr800: fix behavior of set_stereo function

Fix behavior of set_stereo function.

Signed-off-by: David Ellingsworth <david@identd.dyndns.org>
Acked-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>