leds: lm3692x: Use generic support for composing LED names

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

At this occassion remove initialization of struct led_classdev's
dev->of_node property in the driver, since now it will be taken from
fwnode assigned to struct led_init_data and passed to the new
devm_led_classdev_register_ext() API.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Tested-by: Dan Murphy <dmurphy@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>