func_talkdetect.c: Fix logical errors in silence detection.

There are 3 separate changes here:

1. The documentation erroneously stated that the dsp_talking_threshold
   argument was a number of milliseconds when it is actually an energy
   level used by the DSP code to classify talking vs. silence.

2. Fixes a copy paste error in the argument handling code.

3. Don't erroneously switch to the talking state if we aren't actively
   handling a frame we've classified as talking.

Patch inspired by one provided by Moritz Fain (License #6961).

ASTERISK-27816 #close

Change-Id: I5953fd570b98b49c41cee55bfe3b941753fb2511