IB/hfi1: Check eeprom config partition validity

Relying on a trailing magic value is incorrect. There are instances where
this is not present as trailing magic value has a specific purpose which is
not partition validation. Instead use the header magic value which is
present in all variants of the platform configuration and is intended for
validation. This is also used in other locations in the driver.

Fixes: bc5214ee

 (IB/hfi1: Handle missing magic values in config file)
Reviewed-by: Jakub Byczkowski <jakub.byczkowski@intel.com>
Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>