gpio: correct docs about return value of gpiod_get_direction

The use of the GPIOF_* flags is deprecated, so don't advertise them
here. Document the plain numbers for now until we have a better
solution.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -195,7 +195,7 @@ static int gpiochip_find_base(int ngpio)
  * gpiod_get_direction - return the current direction of a GPIO
  * @desc:	GPIO to get the direction of
  *
- * Return GPIOF_DIR_IN or GPIOF_DIR_OUT, or an error code in case of error.
+ * Returns 0 for output, 1 for input, or an error code in case of error.
  *
  * This function may sleep if gpiod_cansleep() is true.
  */