scsi: mpt3sas: always use first reserved smid for ioctl passthrough

ioctl passthrough commands require a SCSIIO smid, but cannot easily
integrate with the block layer. But the driver already has reserved some
SCSIIO smids and we're only ever allowing one ioctl command at a time we
can use the first reserved smid for ioctl commands.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>