[media] cx231xx-input: stop polling if the device got removed.

If the device got removed, stops polling it. Also, un-registers
it at input/evdev, as it won't work anymore. We can't free the
IR structure yet, as the ir_remove method will be called later.

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