leds: el15203000: Introduce to_el15203000_led() helper

Introduce a helper to replace open coded container_of() calls.
At the same time move ldev member to be first in the struct el15203000_led,
that makes container_of() effectivelly a no-op.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>