V4L/DVB (6820): s5h1409: QAM SNR related fixes

QAM SNR values were incorrect when the cable was disconnected. This
patch extends the lookup tables to ensure correct values are being
returned.

Signed-off-by: Steven Toth <stoth@hauppauge.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>