isci: allow fallback to option-rom if efi variable retrieval fails

If the scu efi driver is disabled but the option-rom is enabled (during an efi
boot) allow the code to fallback to scanning legacy option-rom space for the
parameters.

Reported-by: Yinghai Lu <yinghai.lu@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>