leds: lm3642: Use guard to simplify locking

leds: lm3642: Use guard to simplify locking

The mutex_lock()/mutex_unlock() pattern requires explicitly pairing
lock and unlock calls. Use guard(mutex) instead so the lock is
automatically released when the scope exits.

Convert to guard(mutex) in lm3642_torch_brightness_set(),
lm3642_strobe_brightness_set(), and lm3642_indicator_brightness_set().
Add #include <linux/cleanup.h> to support scoped guards.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>
Link: https://patch.msgid.link/20260320035451.31071-1-richard.lyu@suse.com
Signed-off-by: Lee Jones <lee@kernel.org>