scsi: smartpqi: check for null device pointers

- wait on all outstanding I/O to complete before the device is removed.

- check for null device pointers in IO entry/completion functions.

Reviewed-by: Scott Teel <scott.teel@microsemi.com>
Reviewed-by: Murthy Bhat <murthy.bhat@microsemi.com>
Reviewed-by: Dave Carroll <david.carroll@microsemi.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microsemi.com>
Signed-off-by: Mahesh Rajashekhara <mahesh.rajashekhara@microsemi.com>
Signed-off-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>