leds: lp8860: Check return value of devm_mutex_init()

devm_mutex_init() can fail. With CONFIG_DEBUG_MUTEXES=y the mutex will be
marked as unusable and trigger errors on usage.

Add the missed check.

Fixes: 87a59548 ("leds: lp8860: Use new mutex guards to cleanup function exits")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Andrew Davis <afd@ti.com>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250617-must_check-devm_mutex_init-v7-2-d9e449f4d224@weissschuh.net