Documentation: gpio: correct 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>

Documentation/gpio/consumer.txt

@@ -184,7 +184,7 @@ A driver can also query the current direction of a GPIO:
 
 	int gpiod_get_direction(const struct gpio_desc *desc)
 
-This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
+This function returns 0 for output, 1 for input, or an error code in case of error.
 
 Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
 without setting its direction first is illegal and will result in undefined