V4L/DVB (11081): au0828: make sure v4l2_device name is unique

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: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>