gpiolib: use kstrdup_const() for gpio_device label

Users often pass a pointer to a static string to gpiochip_add_data()
family of functions. Avoid unnecessary memory allocations with the
provided helper routine.

While at it: use a ternary operator instead of an if else for brevity.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>