leds: spi-byte: Use devm_mutex_init() for mutex initialization

leds: spi-byte: 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: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240606173037.3091598-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Lee Jones <lee@kernel.org>