[media] au0828: fix race condition that causes xc5000 to not bind for digital

In some cases users would see the xc5000_attach() call failing for the
digital side of the tuner on initialization.  This is because of udev
running v4l-id while the digital side of the board is still coming up.

This is the exact same race condition which was present in em28xx (not
surprising since I copied all the locking logic from that driver when I
added analog support).  Reproduce Mauro's fix from the em28xx driver in
au0828.

Reported-by: Rick Harding <rharding@mitechie.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>