[efi] Add missing RC_TO_EFIRC() conversion

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_snp_hii.c

 			if ( ( rc = efi_snp_hii_fetch ( snpdev, setting->name,
 							NULL, results,
 							NULL ) ) != 0 ) {
-				return rc;
+				return RC_TO_EFIRC ( rc );
 			}
 		}
 	}