ACPI: SPCR: Prefix error messages with FW_BUG

The table values that are not defined in the specification are
considered non-fatal errors in the code.

However, they are firmware bugs, so point this out in the messages by
prefixing them with FW_BUG.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>