usbus: simplify adding entry to list

[keestux]

Contribution description

This is a code simplification albeit that it will now use a pointer-pointer
(which some may claim to add complexity).
The advantage is that no initial if is needed. And it's my believe that
a compiler will make fairly optimized code for it, even with the pointer-pointer.

The original code is this

    usbus_handler_t *last = usbus->handlers;

    if (last) {
        while (last->next) {
            last = last->next;
        }
        last->next = handler;
    }
    else {
        usbus->handlers = handler;
    }

and the new code is this

    usbus_handler_t **last = &usbus->handlers;
    while (*last) {
        last = &(*last)->next;
    }
    *last = handler;

Testing procedure

That particalur USB code is fairly new and only used in cdc_ecm.c. Are there
actual USB tests (besides running some examples)?

Issues/PRs references

The code was recently added in #10916.

[bergzand]

Changes look good, I have some time to test them this evening

[bergzand]

Tested, ACK!