only do the calculations if fax detection is enabled on the dsp.

(closes issue #11331)
Reported by: dimas
Patches:
      dsp.patch uploaded by dimas (license 88)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89490 65c4cc65-6c06-0410-ace0-fbb531ad65f3