gpio: acpi: Fixup kerneldoc

Fix up a parameter description to match the code and fix markup for a
constant to prettify output.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>