usb-linux: Don't try to open the same device twice

If a user wants to redirect 2 identical usb sticks, in theory this is
possible by doing:
usb_add host:1234:5678
usb_add host:1234:5678

But this will lead to us trying to open the first stick twice, since we
don't break the loop after having found a match in our filter list, so the next'
filter list entry will result in us trying to open the same device again.

Fix this by adding the missing break.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
