leds: mlxreg: Use devm_mutex_init() for mutex initialization

leds: mlxreg: Use devm_mutex_init() for mutex initialization

In this driver LEDs are registered using devm_led_classdev_register()
so they are automatically unregistered after module's remove() is done.
led_classdev_unregister() calls module's led_set_brightness() to turn off
the LEDs and that callback uses mutex which was destroyed already
in module's remove() so use devm API instead.

Signed-off-by: George Stark <gnstark@salutedevices.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240411161032.609544-8-gnstark@salutedevices.com
Signed-off-by: Lee Jones <lee@kernel.org>