codec_dahdi: Fix output of "transcoder show" CLI command.

In r306010 "Asterisk media architecture conversion - no more format
bitfields", the logic for incrementing encoders and decoders when
opening transcoder channels was changed without making the corresponding
change when decrementing encoder / decoder channels.  The result being
that when a channel was destroyed, codec_dahdi couldn't properly tell if
it was an encoder or decoder, and the default case is to assume it was a
decoder.

This could result in negative numbers for decoders in use like in:
  VOIP6*CLI> transcoder show
  2/-2 encoders/decoders of 92 channels are in use.

(closes issue ASTERISK-19921)
Patch-by: Shaun Ruffell


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@377382 65c4cc65-6c06-0410-ace0-fbb531ad65f3