LF-5353: dmaengine: imx-sdma: optimize external firmware load check

LF-5353: dmaengine: imx-sdma: optimize external firmware load check

If external firmware has not been loaded, the sdma driver should return
as soon as possible for client using ram script.

The original firmware load check in sdma_transfer_init, if firmware
is not ready, the sdma_config_write also will run, it is unnecessary.

This patch checks firmware load status earlier and return fail immediately
in sdma_config_write if firmware not ready and use ram script.

Signed-off-by: Joy Zou <joy.zou@nxp.com>
Reviewed-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Jason Liu <jason.hui.liu@nxp.com>