IB/hfi1: Add flag for platform config scratch register read

Add flag in pport data structure to determine when platform config was
read from scratch registers. Change conditions in parse_platform_config
and get_platform_config_field to use the new flag.

Reviewed-by: Easwar Hariharan <easwar.hariharan@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Jakub Byczkowski <jakub.byczkowski@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>