leds: leds-lp55xx: Generalize sysfs engine_leds

Generalize sysfs engine_leds since their implementation is the same across
some lp55xx based LED driver.

While at it simplify the implementation for show_engine_leds.

Suggested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://lore.kernel.org/r/20240626160027.19703-17-ansuelsmth@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>