dmaengine: ste_dma40: limit burst size to 16

The client is not aware of the maximum burst size in the dma driver.  If
the size exceeds 16 set max to 16.

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>