mpt3sas: A correction in unmap_resources

It might happen that we try to free an already freed pointer.

Reported-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Acked-by: Chaitra P B <chaitra.basappa@avagotech.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>