leds: mt6360: Get rid of custom led_init_default_state_get()

LED core provides a helper to parse default state from firmware node.
Use it instead of custom implementation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230103131256.33894-8-andriy.shevchenko@linux.intel.com