V4L/DVB: IR/imon: add auto-config for 0xffdc rf device

Add auto-config support for iMON 2.4G LT RF device, based on
debug output from Giulio Amodeo in Red Hat bugzilla #572288.

Also flips the switch on only setting up the rf associate sysfs
attr only if we think we're looking at an RF device, vs. previously,
setting up the attr for all 0xffdc devices, so its possible (but a bit
unlikely) there's another iMON RF device we'll have to fix up.

Nb: should be applied after "IR/imon: clean up usage of bools", or there
will be a slight contextual mismatch.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>