MIPS: lantiq: dma: fix burst length for DEU

The current definition of 2W burst length is invalid.
This patch fixes it. Current downstream DEU driver doesn't
use DMA. An incorrect burst length value doesn't cause any
errors. This patch also adds other burst length values.

Fixes: dfec1a82

 ("MIPS: Lantiq: Add DMA support")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>