gpiolib: Refactor gpiolib_dbg_show() with help of for_each_gpio_desc()

Use for_each_gpio_desc() and since we would need to touch the entire
conditionals, do the following:
- rename last occurrence of gdesc to desc
- use short ternary operator ?:
- join two seq_printf() calls into single one
- fix indentation of the seq_printf() parameters

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