mc1n2: Change DNG threshold from -60 dB to -78 dB

Based on some user testing, a threshold of -60 dB is too loud and causes
DNG to activate in cases where it shouldn't.

That users' results indicate good behavior at -78 dB.  Let's switch to
that for now and see how it works when provided to a wider audience.

http://jira.omnirom.org/browse/OMNI-561

Change-Id: Ic7fe34260f55d645714192e029acf533afe21fb9