trivial: fix typo "thier" -> "their"

 *	and the class driver.

 *

 *	Note: The list is iterated in reverse order, so that we shut down

 *	child devices before we shut down thier parents. The list ordering

 *	child devices before we shut down their parents. The list ordering

 *	is guaranteed by virtue of the fact that child devices are registered

 *	after their parents.

 */