tools/gpio: Add missing initialization of device_name

lsgpio.c: In function ‘main’:
lsgpio.c:166:7: warning: ‘device_name’ may be used uninitialized in this functio
n [-Wmaybe-uninitialized]
   ret = list_device(device_name);
       ^

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>