gpio: make gpiod_is_equal() arguments stricter

gpio: make gpiod_is_equal() arguments stricter

It makes no sense for a GPIO descriptor comparator to return true when
the arguments passed to it are NULL or IS_ERR(). Let's validate both and
return false unless both are valid GPIO descriptors.

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/all/Z_aFBfjb17JxOwyk@black.fi.intel.com/
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/r/20250620-gpiod-is-equal-improv-v1-2-a75060505d2c@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>