V4L/DVB (10641): v4l2-dev: remove limit of 32 devices per driver in get_index()

get_index() had a limitation of 32 devices per driver. This was
unnecessarily strict and has been replaced with the maximum number
of devices. That should really satisfy anyone!

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>