gpio: mockup: Fix potential resource leakage when register a chip

If creation of software node fails, the locally allocated string
array is left unfreed. Free it on error path.

Fixes: 6fda593f

 ("gpio: mockup: Convert to use software nodes")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>