dmaengine: ste_dma40: set dma max seg size

Maximum DMA seg size is (0xffff x data_width).  If max seg
size is not set it deafults to 64k.  This results in failure
if transferring 64k in byte mode.
Large seg sizes may be supported by splitting large transfer.

Signed-off-by: Per Forlin <per.forlin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>