Revert "gpio: remove broken irq_to_gpio() interface"

This reverts commit ee2204a3.

include/linux/gpio.h

@@ -70,6 +70,11 @@ static inline int gpio_to_irq(unsigned int gpio)
 	return __gpio_to_irq(gpio);
 }
 
+static inline int irq_to_gpio(unsigned int irq)
+{
+	return -EINVAL;
+}
+
 #endif /* ! CONFIG_ARCH_HAVE_CUSTOM_GPIO_H */
 
 /* CONFIG_GPIOLIB: bindings for managed devices that want to request gpios */
@@ -217,6 +222,13 @@ static inline void gpiochip_unlock_as_irq(struct gpio_chip *chip,
 	WARN_ON(1);
 }
 
+static inline int irq_to_gpio(unsigned irq)
+{
+	/* irq can never have been returned from gpio_to_irq() */
+	WARN_ON(1);
+	return -EINVAL;
+}
+
 static inline int
 gpiochip_add_pin_range(struct gpio_chip *chip, const char *pinctl_name,
 		       unsigned int gpio_offset, unsigned int pin_offset,