drivers/staging/mt7621-eth: Use dma_zalloc_coherent to replace dma_alloc_coherent+memset

we prefer to use dma_zalloc_coherent rather than dam_alloc_coherent+memset

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>