scsi: megaraid: Use dma_pool_zalloc()

Use dma_pool_zalloc() instead of dma_pool_alloc + memset

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>