dma: imx-sdma: Treat firmware messages as warnings instead of erros

As the SDMA controller can operate without an external firmware being loaded,
treat the firmware related messages as warnings rather than errors.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>