V4L/DVB (10052): radio-mr800: correct unplug, fix to previous patch

This patch corrects unplug procedure, that was implemented wrong in
previous patch. New function usb_amradio_device_release added.
Disconnect lock removed.

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>