leds: leds-gpio: Defer probing in case of deferred gpio probing

This patch makes leds-gpio's probe() return -EPROBE_DEFER if any of the gpios
to register are deferred themselves. This makes a change of
gpio_leds_create_of()'s return value necessary: Instead of returning NULL on
error, we now use ERR_PTR() error coding.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Bryan Wu <cooloney@gmail.com>