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The mutex must be initialized before the LED class device is registered
otherwise there is a race where it may get used before it is initialized:

 DEBUG_LOCKS_WARN_ON(lock->magic != lock)
 WARNING: CPU: 2 PID: 2045 at kernel/locking/mutex.c:587 __mutex_lock
 ...
 RIP: 0010:__mutex_lock+0x7db/0xc10
 ...
 set_brightness_delayed_set_brightness.part.0+0x17/0x60
 set_brightness_delayed+0xf1/0x100
 process_one_work+0x222/0x5a0

Move the mutex_init() call earlier to avoid this race condition and
switch to devm_mutex_init() to avoid the need to add error-exit
cleanup to probe() if probe() fails later on.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240531114124.45346-4-hdegoede@redhat.com

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