leds: mlxreg: Use devm_mutex_init() for mutex initialization

In the Linux kernel, the following vulnerability has been resolved: 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.