leds: lm3530: Ensure drvdata->enable has correct status if regulator_disable fails

Add lm3530_led_enable() and lm3530_led_disable() helper functions.
This ensures setting drvdata->enable to correct status if regulator_disable
fails.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Shreshtha Kumar SAHU <shreshthakumar.sahu@stericsson.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Tested-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>