media: stm32: csi: register subdev only at end of probe

Call v4l2_async_register_subdev only whenever all initialization
are completed at the end of the probe function.
Remove as well useless err_free_priv label by returning directly
upon error.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>