leds: leds-gpio: Fix return value check in create_gpio_led()

In case of error, the function gpio_to_desc() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>