chan_dahdi/PRI: Suppress CONNECTED_LINE updates when nothing in the udpate is valid.

* Also simplified some subddress handling code.

(closes issue ASTERISK-23008)
Reported by: Michael Cargile
........

Merged revisions 405926 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@405927 65c4cc65-6c06-0410-ace0-fbb531ad65f3