dmaengine: imx-sdma: advertise correct residue granularity

The SDMA hardware/driver does not actually report the transfer residue at
burst size granularity, but in fact is only able to report residue after
each finished segment.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>