MIPS: lantiq: dma: add dev pointer

dma_zalloc_coherent() now crashes if no dev pointer is given.
Add a dev pointer to the ltq_dma_channel structure and fill it in the
driver using it.

This fixes a bug introduced in kernel 4.19.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>