leds: class: The -ENOTSUPP should never be seen by user space

Drop the bogus error code and let of_led_get() to take care about absent
of_node.

Fixes: e389240a ("leds: Add managed API to get a LED from a device driver")
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>