leds: lp8860: Use new mutex guards to cleanup function exits

Use scoped mutex guards to simplify return paths. While here use
devm_mutex_init() to register the muxex so it also is cleaned
up automatically.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20250407183555.409687-2-afd@ti.com


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