firmware loader: fix build failure if FW_LOADER is m

device_cache_fw_images need to iterate devices in system,
so this patch applies the introduced dpm_for_each_dev to
avoid link failure if CONFIG_FW_LOADER is m.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>