Fix a bug with the dahdi_setoption callback in chan_dahdi.

This function incorrectly reported success even if the option was
unsupported. This was exposed by the options to change the underlying
channel format. The function now returns a failure if the option
is unsupported.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@188705 65c4cc65-6c06-0410-ace0-fbb531ad65f3