IB/mthca: Use zeroing memory allocator than allocator/memset

Use dma_zalloc_coherent for allocating zeroed
memory and remove unnecessary memset function.

Done using Coccinelle.
Generated-by: scripts/coccinelle/api/alloc/kzalloc-simple.cocci
0-day tested with no failures.

Suggested-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>