gpiolib: Add comments explaining the _cansleep() WARN_ON()s

I've seen users getting very confused by the WARN_ON()s for can_sleep
GPIOs in the atomic-safe paths, the discoverability of the non-atomic
version of the API seems to be hampered by the fact that it's defined
in a header file not the .c file where the warnings are.

Add a couple of comments next to the warnings to help people on their
way.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/gpio/gpiolib.c

@@ -1581,6 +1581,7 @@ int __gpio_get_value(unsigned gpio)
 	int value;
 
 	chip = gpio_to_chip(gpio);
+	/* Should be using gpio_get_value_cansleep() */
 	WARN_ON(chip->can_sleep);
 	value = chip->get ? chip->get(chip, gpio - chip->base) : 0;
 	trace_gpio_value(gpio, 1, value);
@@ -1653,6 +1654,7 @@ void __gpio_set_value(unsigned gpio, int value)
 	struct gpio_chip	*chip;
 
 	chip = gpio_to_chip(gpio);
+	/* Should be using gpio_set_value_cansleep() */
 	WARN_ON(chip->can_sleep);
 	trace_gpio_value(gpio, 0, value);
 	if (test_bit(FLAG_OPEN_DRAIN,  &gpio_desc[gpio].flags))