dma: mv_xor: remove mv_desc_get_dest_addr()

The following commit:

  54f8d501 dmaengine: remove DMA unmap from drivers

removed the last caller to mv_desc_get_dest_addr(), creating the
warning:

  drivers/dma/mv_xor.c:57:12: warning: mv_desc_get_dest_addr defined
  but not used [-Wunused-function]

Remove it.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>