scsi: mpt3sas: Use dma_pool_zalloc

Replaced dma_pool_alloc + memset with dma_pool_zalloc.

Signed-off-by: Brajeswar Ghosh <brajeswar.linux@gmail.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Acked-by: Suganath Prabu <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>