leds: wm8350: Remove work queue

Now the core implements the work queue, remove it from the drivers,
and switch to using brightness_set_blocking op.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Antonio Ospite <ao2@ao2.it>
Reviewed-by: Mark Brown <broonie@kernel.org>