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
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Merged revisions 407337 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 407338 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@407339 f38db490-d61c-443f-a65b-d21fe96a405b