core: Fix tab completion of "core set debug channel" CLI command.

The "core set debug channel" CLI command mistakenly had source filenames
added to its tab completion. This occurred because the CLI generator fell back
to the "core set debug" command which permits setting debug at a source
filename level.

ASTERISK-21038 #close
Reported by: Richard Kenner
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Merged revisions 432944 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432945 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432946 65c4cc65-6c06-0410-ace0-fbb531ad65f3