dmaengine: at_xdmac: fix missing spin_unlock

Lock taken when entering the function but unlock missing before it
returns.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>