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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@405926 f38db490-d61c-443f-a65b-d21fe96a405b