V4L/DVB (10317): radio-mr800: fix radio->muted and radio->stereo

Move radio->muted and radio->stereo in section where radio mutex is
locked to avoid possible race condition problems or access to memory.
Thanks to David Ellingsworth <david@identd.dyndns.org> for pointing to
this weak place in driver.

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>