MIPS: lantiq: Use dma_zalloc_coherent() in dma code

Instead of using dma_alloc_coherent() and memset() directly use
dma_zalloc_coherent().

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19962/
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: john@phrozen.org
Cc: linux-mips@linux-mips.org
Cc: dev@kresin.me