dmaengine: at_xdmac: fix memory leak in interleaved mode

In interleaved mode, when numf > 1, we have only one descriptor for the
transfer but this descriptor has to be added to the descs_list. If not,
when doing remove_xfer, the descriptor won't be put back in the
free_descs_list.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>