dma: mv_xor: use mv_xor_chan pointers as arguments to self-test functions

In preparation for the removal of the mv_xor_device structure, we
directly pass mv_xor_chan pointers to the self-test functions included
in the driver. These functions were anyway selecting the first (and
only channel) available in each DMA device, so the behaviour is
unchanged.

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