wan/fsl_ucc_hdlc: use dma_zalloc_coherent instead of allocator/memset

Use dma_zalloc_coherent instead of dma_alloc_coherent
followed by memset 0.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>