leds: core: Flush scheduled work for system suspend

[ Upstream commit 302a085c20194bfa7df52e0fe684ee0c41da02e6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>