gpio: sim: fix hogs with custom chip labels

We always assign the default device name as the chip_label in hog
structures which makes it impossible to assign hogs to chips. Let's
first check if a custom label was set and then copy it instead of the
default device name.

Fixes: cb8c474e

 ("gpio: sim: new testing module")
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>