bridge_softmix.c,channel.c: Minor code simplification and cleanup.

* Made code easier to follow in bridge_softmix.c:analyse_softmix_stats()
and made some debug messages more helpful.

* Made some debug and warning messages more helpful in
channel.c:set_format().

Review: https://reviewboard.asterisk.org/r/4607/
........

Merged revisions 434617 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434618 65c4cc65-6c06-0410-ace0-fbb531ad65f3