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
........

Merged revisions 377382 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@377383 65c4cc65-6c06-0410-ace0-fbb531ad65f3

codecs/codec_dahdi.c

@@ -394,7 +394,7 @@ static void dahdi_destroy(struct ast_trans_pvt *pvt)
 {
 	struct codec_dahdi_pvt *dahdip = pvt->pvt;
 
-	switch (dahdip->fmts.dstfmt) {
+	switch (ast_format_id_from_old_bitfield(dahdip->fmts.dstfmt)) {
 	case AST_FORMAT_G729A:
 	case AST_FORMAT_G723_1:
 		ast_atomic_fetchadd_int(&channels.encoders, -1);