leds: Introduce ordered workqueue for LEDs events instead of system_wq

This allows to setup ordered workqueue for LEDs events. This may be
useful, because default 'system_wq' does not guarantee execution order
of each work_struct, thus for several brightness update requests (for
multiple LEDs), real brightness switch could be in random order.

Yes, for sysfs-based LEDs we have flush_work() call inside
brightness_store() operation, but it's blocking call, so userspace
caller can be blocked at a long time, which means LEDs animation stream
can be broken.

Ordered workqueue has the same behaviour as system_wq + flush_work(),
but all scheduled works are async and userspace caller is not blocked,
which it better for userspace animation scheduling.

Signed-off-by: Alexey Romanov <[email protected]>
Signed-off-by: Dmitry Rokosov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[Lee: Couple of style fix-ups]
Signed-off-by: Lee Jones <[email protected]>

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