The Crystal Cove PMIC has 16 real GPIOs but the ACPI code for devices
with this PMIC may address up to 95 GPIOs, these extra GPIOs are
called virtual GPIOs and are used by the ACPI code as a method of
accessing various non GPIO bits of PMIC.

Commit dcdc3018 ("gpio: crystalcove: support virtual GPIO") added
dummy support for these to avoid a bunch of ACPI errors, but instead of
ignoring writes / reads to them by doing:

if (gpio >= CRYSTALCOVE_GPIO_NUM)
	return 0;

It accidentally introduced the following wrong check:

if (gpio > CRYSTALCOVE_VGPIO_NUM)
	return 0;

Which means that attempts by the ACPI code to access these gpios
causes some arbitrary gpio to get touched through for example
GPIO1P0CTLO + gpionr % 8.

Since we do support input/output (but not interrupts) on the 0x5e
virtual GPIO, this commit makes to_reg return -ENOTSUPP for unsupported
virtual GPIOs so as to not have to check for (gpio >= CRYSTALCOVE_GPIO_NUM
&& gpio != 0x5e) everywhere and to make it easier to add support for more
virtual GPIOs in the future.

It then adds a check for to_reg returning an error to all callers where
this may happen fixing the ACPI code accessing virtual GPIOs accidentally
causing changes to real GPIOs.

Fixes: dcdc3018

 ("gpio: crystalcove: support virtual GPIO")
Cc: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>