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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@407337 f38db490-d61c-443f-a65b-d21fe96a405b