dmaengine: at_xdmac: clean used descriptor

When putting back a descriptor to the free descs list, some fields are
not set to 0, it can cause bugs if someone uses it without having this
in mind.
Descriptor are not put back one by one so it is easier to clean
descriptors when we request them.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable@vger.kernel.org #4.2
Signed-off-by: Vinod Koul <vinod.koul@intel.com>