media: dw9807-vcm: Fix probe error handling

v4l2_async_unregister_subdev() may not be called without
v4l2_async_register_subdev() being called first. Fix this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>