ibmvscsi: set max_lun to 32

As defined in 4.6.9 of SAM-4, the encoding of LUN is
on 5 bits (max_lun=32) and the current value is only 8.
Set max_lun to IBMVSCSI_MAX_LUN (32).

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>