ARM: OMAP1: ams-delta: fix early boot crash when LED support is disabled

When we boot with the LED support (CONFIG_NEW_LEDS) disabled,
gpio_led_register_device() will return a NULL pointer and we try
to dereference it. Fix by checking also for a NULL pointer.

Fixes: 19a2668a ("ARM: OMAP1: ams-delta: Provide GPIO lookup table for LED device")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>