leds: pwm-multicolor: Use fwnode_get_child_node_count()

Since fwnode_get_child_node_count() was split from its device property
counterpart, we may utilise it in the driver and drop custom implementation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20250310150835.3139322-3-andriy.shevchenko@linux.intel.com


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