dma: xilinx: zynqmp_dma: Fix race condition in the driver

dma: xilinx: zynqmp_dma: Fix race condition in the driver

In the driver software descriptor pools are allocated only when
SG is enabled in the driver. but we are freeing the desc polls in
Simple dma mode case also which is causing kernel crash when running
The dmatest client multiple times this patch fixes this issue.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri<punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>