dmaengine i.MX SDMA: set firmware scripts addresses to negative value initially

If we do not have a firmare script for a given transfer,
the setup of this channel must fail. For this the script
addresses have to be < 0 initially, not 0.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>