leds-lp5523: set the brightness to 0 forcely on removing the driver

Turning off the brightness of each channel is required
when removing the driver.

So use flush_work() rather than cancel_work_sync() to execute
remaining brightness works.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>