Unmute when we get a dtmfup (we muted on dtmfdown) event.

This would occasionally cause one-way audio when using hardware DTMF detection.

(closes issue #14761)
Reported by: tzafrir
Patches:
      v1-14761.patch uploaded by dimas (license 88)
Tested by: tzafrir, dimas


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@203258 65c4cc65-6c06-0410-ace0-fbb531ad65f3