net-next: mediatek: improve more with using dma_zalloc_coherent

Improve more in the existing code by reusing dma_zalloc_coherent instead
of dma_alloc_coherent with __GFP_ZERO or superfluous zeroing buffer.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>