[media] uvcvideo: Fix open/close race condition

Maintaining the users count using an atomic variable makes sure that
access to the counter won't be racy, but doesn't serialize access to the
operations protected by the counter. This creates a race condition that
could result in the status URB being submitted multiple times.
Use a mutex to protect the users count and serialize access to the
status start and stop operations.

Reported-by: Shawn Nematbakhsh <shawnn@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>