leds: leds-lp55xx: Convert mutex lock/unlock to guard API

Convert any entry of mutex lock/unlock to guard API and simplify code.
With the use of guard API, handling for selttest functions can be
greatly simplified.

Suggested-by: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://lore.kernel.org/r/20240626221520.2846-3-ansuelsmth@gmail.com


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