gpiolib: Simplify error path in gpiod_get_index() when requesting GPIO

Simplify error path in the gpiod_get_index() when requesting a GPIO line by:
 - checking for error condition first
 - dropping redundant 'else'

As a result, decrease the indentation level for better readability.

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