chan_dahdi: Fix analog dialtone detection.

* Check if waitingfordt (waitfordialtone) is enabled in dahdi_read() to
allow the DSP to operate early enough to detect dialtone.

* Made use the correct variable in my_check_waitingfordt().

ASTERISK-23709 #close
Reported by: Steve Davies
Patches:
      dialtone_detect_fix (license #5012) patch uploaded by Steve Davies

Review: https://reviewboard.asterisk.org/r/3534/
........

Merged revisions 414067 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@414068 65c4cc65-6c06-0410-ace0-fbb531ad65f3