gpio: improve the API contract for setting direction

If a GPIO driver returns a positive integer from one of the direction
setter callbacks, we'll end up propagating it to user-space. Whether we
should sanitize the values returned by callbacks is a different question
but let's first improve the documentation and fortify the contract with
GPIO providers.
Reported-by: José Guilherme de Castro Rodrigues <joseguilhermebh@hotmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>

include/linux/gpio/driver.h

@@ -335,10 +335,12 @@ struct gpio_irq_chip {
  *	(same as GPIO_LINE_DIRECTION_OUT / GPIO_LINE_DIRECTION_IN),
  *	or negative error. It is recommended to always implement this
  *	function, even on input-only or output-only gpio chips.
- * @direction_input: configures signal "offset" as input, or returns error
- *	This can be omitted on input-only or output-only gpio chips.
- * @direction_output: configures signal "offset" as output, or returns error
- *	This can be omitted on input-only or output-only gpio chips.
+ * @direction_input: configures signal "offset" as input, returns 0 on success
+ *	or a negative error number. This can be omitted on input-only or
+ *	output-only gpio chips.
+ * @direction_output: configures signal "offset" as output, returns 0 on
+ *	success or a negative error number. This can be omitted on input-only
+ *	or output-only gpio chips.
  * @get: returns value for signal "offset", 0=low, 1=high, or negative error
  * @get_multiple: reads values for multiple signals defined by "mask" and
  *	stores them in "bits", returns 0 on success or negative error