[media] bttv: merge ir decoding timers

Similarly to saa7134, bttv_ir has two timers, only one of which is used
at a time and which serve the same purpose. Merge them.

Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>