[media] radio-timb: actually load the requested subdevs

For some reason the tuner and dsp subdevs were never actually loaded.
Added the relevant code to do that.
Also remove bogus calls to video_device_release_empty().

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Richard Röjfors <richard.rojfors@pelagicore.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>