[media] usbtv: clarify the licensing

OpenBSD would like to reuse some code but consider the licensing not
clear enough. Let's clarify it a bit so that it suits their conventions:

1.) Keep the "extra text" away from the copyright statement and the
rights grant.

2.) Add the warranty disclaimer -- it should not be legally required,
nevertheless the clause 1. of the rights grant refest to it.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Federico Simoncelli <fsimonce@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>