Fixes issue with undefined audio codecs in chan_iax2

Fixes issue with undefined audio codecs in chan_iax2

During iax2 call negotiation, supported codecs are passed in an Information Element containing a 2 byte field where each bit correlates to a specific codec.  In 1.4 only audio codec bits 0-12 are defined, leaving bits 13-15 undefined.  By default all bits are enabled unless specified otherwise.  Since its a 2 byte field and 13-15 are not defined, these bits are never turned off.  In trunk, bits 13-15 are defined, which means 1.4 is advertising support for codecs it does not have when talking to trunk.  I fixed this by adding #define for undefined audio codec bits.  These bits are then removed from iax2's full bandwidth capabilities.

(closes issue #14283)
Reported by: jcovert

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@177696 65c4cc65-6c06-0410-ace0-fbb531ad65f3