power supply : use class iteration api

Convert to use the class iteration api.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Cc: Anton Vorontsov <cbou@mail.ru>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>