devicestate: Make ast_devstate_changed_literal() return value and doxygen consistent.

Nothing actually cares about the value anyway.

(closes issue ASTERISK-23178)
Reported by: Jonathan Rose
........

Merged revisions 407337 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@407338 65c4cc65-6c06-0410-ace0-fbb531ad65f3