Cothread: Fix bug in DBR_CHAR_STR and DBR_CHAR_UNICODE conversion

As of git commit 3e567683 the option datatype=DBR_CHAR_STR or
DBR_CHAR_UNICODE fails to return a value because of a refactoring error.
This fix fixes this problem.

git-svn-id: svn+ssh://serv0002.cs.diamond.ac.uk/home/subversion/repos/controls/diamond/trunk/python/cothread@114956 e099a375-04f9-0310-9d5f-a741eaff62e1