Make sure newly created v4l2 devices have a unique name, modeling the logic
after the cx18 driver.

Thanks to Andy Walls <awalls@radix.net> for pointing out the issue.
Signed-off-by: NDevin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>