[media] rc-core: lirc use unsigned int

Durations can never be negative, so it makes sense to consistently use
unsigned int for LIRC transmission. Contrary to the initial impression,
this shouldn't actually change the userspace API.

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