dmaengine: dw: set cdesc to NULL when free cyclic transfers

To be sure we have the cyclic transfers already gone we set cdesc to NULL. It
will prevent the double free.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>