dmaengine: at_xdmac: fix a bug in transfer residue computation

The total size of the transfer was wrong in at_xdmac_prep_slave_sg()
resulting in bad computation of the transfer residue by
at_xdmac_tx_status().

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