Fix following sparse warning:

drivers/scsi/mvumi.c:1797:48: warning: Using plain integer as NULL pointer
drivers/scsi/mvumi.c:2143:50: warning: Using plain integer as NULL pointer
drivers/scsi/mvumi.c:755:58: warning: Using plain integer as NULL pointer
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>