dmaengine: imx-sdma: sdma_get_firmware does not need to copy fw_name

It does not need to allocate space and copy fw_name in function
sdma_get_firmware().

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>