gpiolib: Use short form of ternary operator in gpiod_get_index()

Instead of repeating first argument for true branch, use short
form of the ternary operator, i.e. ?:.

While at it, fix a typo in the comment.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>