libkmod: use uint8_t for the child prefix/index

libkmod: use uint8_t for the child prefix/index

Stop implicitly casting the child prefix/index to int. It can have high
bits set thus get promoted to wildly incorrect value and cause chaos
further on.

In addition, convert the existing `unsigned char` instances to uint8_t,
which better illustrates what we're after - a fixed sized 8 bit unsigned
integer.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/251
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>