dma: mv_xor: Rename __mv_xor_slot_cleanup() to mv_xor_slot_cleanup()

Now that mv_xor_slot_cleanup() has no remaining callers, we remove it
and rename __mv_xor_slot_cleanup() to mv_xor_slot_cleanup().

We take this opportunity to add a comment that makes it clear that the
channel spinlock should be held before calling mv_xor_slot_cleanup().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>