V4L/DVB (10460): radio-mr800: add stereo support

Patch introduces new amradio_set_stereo function.
Driver calls this func to make stereo radio reception.
Corrects checking of returned value after usb_bulk_msg.

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>