Fix IAX2 registration time. We want to set reg->refresh to the new value only...

Fix IAX2 registration time.  We want to set reg->refresh to the new value only if its lower not if its higher. (bug 1955)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@3369 65c4cc65-6c06-0410-ace0-fbb531ad65f3