Move the bus offset configuration discovery code to a new helper.  Modify
the bus offset 2-bit decode switch to have a 0 case and a default error
case, just in case the field is expanded in future hardware.

Link: https://lore.kernel.org/r/20200728194945.14126-3-jonathan.derrick@intel.comSigned-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>