A county prosecutor in Massachusetts has been fired and charged with falsifying evidence against a cocaine dealer in an effort to discredit the presidential campaign of Democratic Gov. Michael S. Dukakis. Kevin Ross, 33, of Waltham, Mass., a former Suffolk County assistant district attorney, allegedly slipped a phony address book containing the names of several Dukakis advisers in with evidence collected against the dealer. He was fired after allegedly confessing and was released on $10,000 bail after a hearing in U.S. District Court. Ross, who allegedly intended to discredit Dukakis because he felt the governor was soft on crime, inserted the names of Dukakis campaign chairman Paul P. Brountas, chief fund-raiser Robert Farmer and several low-level Dukakis workers into the book, prosecutors said.