gpiolib: Deduplicate cleanup for-loop in gpiochip_add_data_with_key()

There is no need to repeat for-loop twice in the error path in
gpiochip_add_data_with_key(). Deduplicate it. While at it,
rename loop variable to be more specific and avoid ambguity.

It also properly unwinds the SRCU, i.e. in reversed order of allocating.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>