dmaengine: at_hdmac: fix buffer transfer size specification

Buffer transfer size is the number of transfers to be performed in
relation with the width of the _source_ interface.
So in the DMA_FROM_DEVICE case, it should be the register width that
should be taken into account.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>