V4L/DVB: dib0700: Fix RC protocol logic to properly handle NEC/NECx and RC-5

Simplifies the logic for handling firmware 1.20 RC messages, fixing the
logic.

While here, I tried to use a RC-6 remote controller from my TV set, but it
didn't work with dib0700. Not sure why, but maybe this never worked.

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