video_register_device_index is never actually called, instead the
stream index number is always calculated automatically.
This patch removes this function and simplifies the internal get_index
function since that can now always just return the first free index.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>