strength and snr dvb values should be unsigned.

The problem with the wrong types was reported by Dr. Torge Szczepanek,
and after asking on linux-media mailing list it turns out that he was
right and the documentation is wrong:

http://www.mail-archive.com/linux-media@vger.kernel.org/msg70287.html

This does not change the comm protocol, just the interpretation of
the returned values.