scsi: megaraid_sas: fix error handle in megasas_probe_one

megasas_mgmt_info.max_index has increased by 1 before megasas_io_attach,
if megasas_io_attach return error, then goto fail_io_attach,
megasas_mgmt_info.instance has a wrong index here. So first reduce
max_index and then set that instance to NULL.

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>