[media] radio-mr800: add hardware seek support

Added hardware seek support based on information gleaned from the
GPLv2 driver available here:
http://sourceforge.net/projects/av-usbradio/Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>