leds: core: Flush scheduled work for system suspend

Sometimes LED won't be turned off by LED_CORE_SUSPENDRESUME flag upon
system suspend.

led_set_brightness_nopm() uses schedule_work() to set LED brightness.
However, there's no guarantee that the scheduled work gets executed
because no one flushes the work.

So flush the scheduled work to make sure LED gets turned off.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Fixes: 81fe8e5b ("leds: core: Add led_set_brightness_nosleep{nopm} functions")
Signed-off-by: Pavel Machek <pavel@ucw.cz>