net: ethernet: ixp4xx: Make use of dma_pool_zalloc() instead of dma_pool_alloc/memset()

Replacing dma_pool_alloc/memset() with dma_pool_zalloc()
to simplify the code.

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>