dma: mv_xor: introduce a mv_chan_to_devp() helper

In many place, we need to get the 'struct device *' pointer from a
'struct mv_chan *', so we add a helper that makes this a bit
easier. It will also help reducing the change noise in further
patches.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>