leds: lm3642: Use regmap_update_bits() in lm3642_chip_init()

Use regmap_update_bits() to replace regmap_read() + regmap_write().
With this patch, we only show the error message when regmap_update_bits()
fails.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: G.Shark <Jeong&lt;gshark.jeong@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>