media: dw9714: Fix error handling in probe function

Fixed the case where v4l2_async_unregister_subdev()
is called unnecessarily in the error handling path
in probe function.

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