Fix getChannelUserModes().

It was apparently meant to return prefixes. Who knew.

Change-Id: I3dd5bde3b0902fdaa690de8b8352380d5731c8f0
Reviewed-on: http://gerrit.dmdirc.com/3990
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>