dahdi_read relies on the dahdi_pvt copy of ringt which was not getting set
in sig_analog. This patch adds a callback to do so.

(closes issue #15288)
Reported by: alecdavis
Patches:
      chan_dahdi.ringtimeout.diff.txt uploaded by alecdavis (license 585)
Tested by: alecdavis



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@211908 65c4cc65-6c06-0410-ace0-fbb531ad65f3