leds: as3645a: Use generic support for composing LED names

Switch to using generic LED support for composing LED class
device name.

After switching to new led_classdev_register_ext() the validity
of struct led_classdev's name property is no longer guaranteed,
and therefore rely on struct device's kobj.name instead.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>