dmaengine: xilinx_dma: Fix race condition in the driver for multiple descriptor scenario

dmaengine: xilinx_dma: Fix race condition in the driver for multiple descriptor scenario

As per AXI DMA spec the software must not move the tail pointer to a location
That has not been updated (next descriptor field of the h/w descriptor
Should always point to a valid address).

When user submits multiple descriptors on the recv side, with the
Current driver flow the last buffer descriptor next descriptor field
Points to a invalid location, resulting the invalid data or errors in the
DMA engine.

This patch fixes this issue by creating a Buffer Descritpor Chain during
Channel allocation itself and use those Buffer Descriptors.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>