The table of valid SNR values for QAM 256 is off by one, and as a result
if the SNR is oscillating between 40.0 and 39.9 dB, tools like azap show
it going back and forth between 40.0 and 0 (misleading some people,
including myself, to think signal lock is being lost or there is a
problem with register reads).

Fix the table so that 40.0 dB is properly represented.

Cc: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>