dmaengine: imx-dma: Disable use of hw_chain to fix sg_dma transfers.

HW chaining is currently broken in imx-dma. It can be easily reproduced doing
intensive accesses to a external MMC card and checking how the file system
is corrupted.

Preventing the driver to use HW chaining solves these issues.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>