gpio: constify arguments of gpiod_is_equal()

This function is not meant to modify the GPIO descriptors in any way so
we can safely constify both arguments.

Link: https://lore.kernel.org/r/20250620-gpiod-is-equal-improv-v1-1-a75060505d2c@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpiolib.c

@@ -286,7 +286,7 @@ EXPORT_SYMBOL_GPL(gpiod_to_gpio_device);
  * Returns:
  * True if the descriptors refer to the same physical pin. False otherwise.
  */
-bool gpiod_is_equal(struct gpio_desc *desc, struct gpio_desc *other)
+bool gpiod_is_equal(const struct gpio_desc *desc, const struct gpio_desc *other)
 {
 	return desc == other;
 }

include/linux/gpio/consumer.h

@@ -181,7 +181,8 @@ struct gpio_desc *devm_fwnode_gpiod_get_index(struct device *dev,
 					      enum gpiod_flags flags,
 					      const char *label);
 
-bool gpiod_is_equal(struct gpio_desc *desc, struct gpio_desc *other);
+bool gpiod_is_equal(const struct gpio_desc *desc,
+		    const struct gpio_desc *other);
 
 #else /* CONFIG_GPIOLIB */
 
@@ -551,7 +552,7 @@ struct gpio_desc *devm_fwnode_gpiod_get_index(struct device *dev,
 }
 
 static inline bool
-gpiod_is_equal(struct gpio_desc *desc, struct gpio_desc *other)
+gpiod_is_equal(const struct gpio_desc *desc, const struct gpio_desc *other)
 {
 	WARN_ON(desc || other);
 	return false;