ACPI: power: Use u8 as the power resource state data type

Use u8 as the data type for representing the state of an ACPI
power resource.

It is s not necessary to use int for that and because subsequent
changes are going to use ACPI_POWER_RESOURCE_STATE_UNKNOWN, it is
better to adjust the data type so that the "unknown" state is
represented by the "all ones" value.

While at it, clean up acpi_power_get_state() somewhat.

No intentional functional impact.

Tested-by: Dave Olsthoorn <dave@bewaar.me>
Tested-by: Shujun Wang <wsj20369@163.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>