gpio: return NULL from gpiod_get_optional when GPIOLIB is disabled

Given the intent behind gpiod_get_optional() and friends it does not make
sense to return -ENOSYS when GPIOLIB is disabled: the driver is expected to
work just fine without gpio so let's behave as if gpio was not found.
Otherwise we have to special-case -ENOSYS in drivers.

Note that there was objection that someone might forget to enable GPIOLIB
when dealing with a platform that has device that actually specifies
optional gpio and we'll break it. I find this unconvincing as that would
have to be the *only GPIO* in the system, which is extremely unlikely.

Signed-off-by: Dmitry Torokhov <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>

Documentation/gpio/consumer.txt

@@ -70,6 +70,12 @@ instead of -ENOENT if no GPIO has been assigned to the requested function:
 						   unsigned int index,
 						   enum gpiod_flags flags)
 
+Note that gpio_get*_optional() functions (and their managed variants), unlike
+the rest of gpiolib API, also return NULL when gpiolib support is disabled.
+This is helpful to driver authors, since they do not need to special case
+-ENOSYS return codes.  System integrators should however be careful to enable
+gpiolib on systems that need it.
+
 For a function using multiple GPIOs all of those can be obtained with one call:
 
 	struct gpio_descs *gpiod_get_array(struct device *dev,