dma: xilinx: Fix race conditions in the driver for cdma

dma: xilinx: Fix race conditions in the driver for cdma

This patch fixes the below issues
--> when hardware is idle we need to toggle the SG bit
inorder to update new value to the current descriptor
register other wise undefined results will occur.
--> Halt bit is not valid for cdma case add checks
for the same.

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