leds: pwm: Simplify with resource-managed devm_led_classdev_register()

Simplify the exit path with resource-managed version of registering LED
class device.  The code should be functionally the same, except that on
device removal the led_pwm_priv->num_leds is not decremented to zero
(which should not have any effect as device is going away).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>