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