dma: imx-sdma: Let the driver be probed even if no firware is found

On the device tree case the code bails out when the firmware name cannot be found
or if the firmware fails to be requested.

Fix this behaviour as the SDMA engine can still operate with the built-in
ROM scripts.

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>