V4L/DVB (10458): radio-mr800: move radio start and stop in one function

Patch introduces new amradio_set_mute function. Amradio_start and
amradio_stop removed. This makes driver more flexible and it's useful
for next changes.

Signed-off-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>