Use TRUNK_CALL_START as originally intended.

Back in r646, TRUNK_CALL_START was added and defined as 0x4000.  That same value
was also hard-coded in one part of the IAX2 code instead of using the #define.

TRUNK_CALL_START has changed over the years (for dealing with LOW_MEMORY), but
the hard-coded usage was never updated to match.  This patch fixes that.
........

Merged revisions 355448 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 355449 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@355450 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_iax2.c

@@ -3507,7 +3507,7 @@ retry:
 		ast_channel_unlock(owner);
 	}
 
-	if (callno & 0x4000) {
+	if (callno & TRUNK_CALL_START) {
 		update_max_trunk();
 	}
 }