MLK-25041: media: imx8: media-dev: return if media device has been registered

commit 985c60e55694a69effd2197f798cb039a17144c6 from
https://source.codeaurora.org/external/imx/linux-imx



subdev_notifier_complete will be called multi times in some cases
due to workaround to support hotplugin-like mechanism, so add checking
media device register status before register new media device.

Signed-off-by: Guoniu.zhou <guoniu.zhou@nxp.com>
Reviewed-by: Robby Cai <robby.cai@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>