Use TRUNK_CALL_START as originally intended.

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@355449 65c4cc65-6c06-0410-ace0-fbb531ad65f3