dma: xilinx: axidmatest: Fix race conditions in the test client

dma: xilinx: axidmatest: Fix race conditions in the test client

This patch does the below
--> Test client is limiting the test buffer size to 700
remove the check for the same also this patch increases the
default value of test buf size from 64 to 16k.
--> Terminate the existing transactions before freeing the dma channel.

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