leds-lp55xx: use lp55xx_set_brightness()

leds-lp55xx: use lp55xx_set_brightness()

 lp5521_set_brightness() and lp5523_set_brightness() are replaced with
 common function, lp55xx_set_brightness().
 This function is invoked when the brightness of each LED channel is updated.
 LP5521 and LP5523 have different register address for the brightness control,
 so this work is done by chip specific brightness_work_fn().

 lp5521/5523_led_brightness_work():
 use lp55xx_led and lp55xx_chip data structure.
 use lp55xx write function.

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