dsp.c fix incorrect DTMF Digit_Duration.

it's always short by 'hits_to_begin*DTMF_GSIZE', or 25.5ms if hitstobegin=2

(issue ASTERISK-16003)
Tested by: alecdavis
alecdavis (license 585)

Review https://reviewboard.asterisk.org/r/2145/
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Merged revisions 374475 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374476 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374477 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374478 65c4cc65-6c06-0410-ace0-fbb531ad65f3