firmware loader: sync firmware cache by async_synchronize_full_domain

async.c has provided synchronization mechanism on async_schedule_*,
so use async_synchronize_full_domain to sync caching firmware instead
of reinventing the wheel.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>