dmaengine: at_xdmac: fix false condition for memset_sg transfers

The code was not in agreement with the comments.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable@vger.kernel.org # 4.3 and later
Signed-off-by: Vinod Koul <vinod.koul@intel.com>