media: imx: imx7-media-csi: Cleanup errors in imx7_csi_async_register()

It's good practice for functions to perform error cleanup internally
when they fail, in order to not leave the device in a half-initialized
state. Move the async notifier cleanup from the probe error path to the
imx7_csi_async_register(), and drop the v4l2_async_nf_unregister() call
as there is no error path after the async notifier gets registered.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm-beacon-kit
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>