Fix sending of interface identifier unconditionally in sig_pri

The wrong logic was being used in chan_dahdi to convert a sig_pri_chan
to the proper libpri channel number. The most significant bit must only
be set only when trunk groups are being used.

(closes issue #15452)
Reported by: alecdavis
Patches:
      bug15452.patch uploaded by jpeeler (license 325)
Tested by: alecdavis



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@208267 65c4cc65-6c06-0410-ace0-fbb531ad65f3