lib: smf: use `while (true)` in `get_child_of` for better clarity

Replace the condition-less `for` loop (`;;`) in `get_child_of` with
a `while (true)` loop and remove the redundant `return NULL;`
at the end, which is never reached.

This change aims to enhance readability since `while (true)` is better
suited for this scenario.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>