smf: use get_child_of() for topmost in smf_set_initial()

Replace get_last_of(init_state) with get_child_of(init_state, NULL),
and add a brief comment to avoid relying on a trivial wrapper. Also,
the wrapper name get_last_of() is not very descriptive; using
get_child_of() with an comment makes the intent more explicit.

Signed-off-by: Vladislav Kulikov <vlad_kulikov_c@pm.me>