V4L/DVB (13073): radio-mr800: set radio frequency only upon success

Set radio frequency only upon success.

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>