core: Narrow the types passed to certain backend functions

core: Narrow the types passed to certain backend functions

Backend functions dealing with interfaces and alternate settings should
use a type whose range represents that of valid values for interfaces
and alternate settings. Switch to use uint8_t instead of int so that
backends do not have to cast values or do range checks.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>