net: ixp4xx_hss: use dma_pool_zalloc

The dma_pool_zalloc combines dma_pool_alloc/memset. Therefore, the
dma_pool_alloc/memset can be replaced with dma_pool_zalloc which is
more compact.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>