V4L/DVB (9628): em28xx: refactor IR support

Refactor the em28xx IR support based on the em2860/em2880 and em2874
datasheets.

Tested on the HVR-950 (em2883), Pinnacle 800e (em2883), Pinnacle 80e (em2874)
using the remote controls that came with those products.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>