gpiolib: acpi: Convert type for pin to be unsigned

A pin that comes from ACPI tables is of unsigned type. This also applies
to the internal APIs which use unsigned int to store the pin. Convert
type for pin to be unsigned in the places where it's not yet true.

While at it, add a stub for acpi_get_and_request_gpiod() for the sake
of consistency in the APIs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>