chan_iax2: Restore previous behavior of iax2_best_codec.
The iax2_best_codec function was changed to convert the formats
into a format compatibilities structure and grab the first
format from it. The resulting order differs from the previous
order of iax2_best_codec which causes unexpected formats to
get chosen (such as g723).
This commit brings back the old behavior of iax2_best_codec by
having a specified preference list.
Review: https://reviewboard.asterisk.org/r/3835/
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419180
65c4cc65-6c06-0410-ace0-
fbb531ad65f3