leds: lm3642: Remove work queue

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

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Cc: Daniel Jeong <daniel.jeong@ti.com>
Cc: G.Shark Jeong <gshark.jeong@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>