PM: sleep: core: Switch back to async_schedule_dev()

Commit 8b9ec6b7 ("PM core: Use new async_schedule_dev command")
introduced a new function for better performance.

However commit f2a424f6 ("PM / core: Introduce dpm_async_fn()
helper") went back to the non-optimized version, async_schedule().

So switch back to the sync_schedule_dev() to improve performance

Fixes: f2a424f6 ("PM / core: Introduce dpm_async_fn() helper")
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>