gpio_led: check gpio_request() return value

Add a check for the gpio_request() function return value and do not try
to configure the GPIO if the gpio_request() call fails.
Also, print an error message indicating the gpio_request() has failed.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Tested-by: Ilya Ledvich <ilya@compulab.co.il>