scsi: mvsas: Use dma_pool_zalloc

Replace dma_pool_alloc + memset with dma_pool_zalloc.

Signed-off-by: Sabyasachi Gupta <sabyasachi.linux@gmail.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>