gpiolib: Even more opportunities to use str_high_low() helper

gpiolib: Even more opportunities to use str_high_low() helper

The one of previous changes modified the library code to use
helpers from string_choices.h. Nevertheless it misses more
opportunities to convert the code. Here is the second part
of the conversion.

Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250205112936.575493-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>