DMA: AT91: Get transfer width

In one dma transfer, the data transfer width can be configured and it is limited by source or destination peripheral width,
tx_width will save the transfer width, but for memcpy, either source or destination transfer width is taken as tx_width.

Signed-off-by: Elen Song <elen.song@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>