dma: mv_xor: Remove unneeded mv_xor_clean_completed_slots() call

In mv_xor_status(), we are currently calling mv_xor_clean_completed_slots()
when the transaction is complete (the cookie status is DMA_COMPLETE).
However, a completed status means that mv_xor_slot_cleanup() was called,
which cleans the completed slots.

In other words, there's nothing to cleanup for a completed transaction in
mv_xor_status(). Remove the unneeded call to mv_xor_clean_completed_slots().

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