dsp.c: optimize goerztzel sample loops, in dtmf_detect, mf_detect and tone_detect

dsp.c: optimize goerztzel sample loops, in dtmf_detect, mf_detect and tone_detect

use a temporary short int when repeatedly used to call goertzel_sample.

alecdavis (license 585)
Reported by: alecdavis
Tested by: alecdavis

Review: https://reviewboard.asterisk.org/r/2093/
........

Merged revisions 372212 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 372213 from http://svn.asterisk.org/svn/asterisk/branches/10
........

Merged revisions 372214 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372215 65c4cc65-6c06-0410-ace0-fbb531ad65f3