drivers: led_pwm: fix device name fallback

If the label property is missing in a "pwm-leds" compatible DT node
(which is almost always the case), then the device name is now set to
DT_NODE_FULL_NAME instead of "LED_PWM_$inst" previously.

This allows applications to use the DEVICE_DT_NAME macro to retrieve
the device name instead of gessing an arbitrary string.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>