Update the the dmatest driver so that it handles duplicate DMA channels
properly.

When a DMA client is notified of an available DMA channel, it must check if it
has already allocated resources for that channel.  If so, it should return
DMA_DUP.  This can happen, for example, if a DMA driver calls
dma_async_device_register() more than once.
Acked-by: NHaavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: NTimur Tabi <timur@freescale.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>