media: uvcvideo: Fix driver reference counting

kref_init initializes the reference count to 1, not 0. This additional
reference is never released since the conversion to reference counters.
As a result, uvc_delete is not called anymore when UVC cameras are
disconnected.
Fix this by adding an additional kref_put in uvc_disconnect and in the
probe error path. This also allows to remove the temporary additional
reference in uvc_unregister_video.

Fixes: 9d15cd95 ("media: uvcvideo: Convert from using an atomic variable to a reference count")

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>