dma: mv_xor: in mv_xor_chan, rename 'common' to 'dmachan'

The mv_xor_chan structure embeds a 'struct dma_chan', which is named
'common', a not very meaningful name. Rename it to 'dmachan', which
will help avoid confusions later as we merge the mv_xor_device and
mv_xor_chan structures together.

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