By Richard Cowan WASHINGTON (Reuters) -Negotiators on a U.S. Senate gun control bill could finish writing the text of the legislation later this week, Republican Senator John Cornyn said on Tuesday, adding that doing so could clear it for votes by the full Senate as early as next week. Cornyn, the main Republican negotiator on a bipartisan bill that would expand gun sale background checks and support states that institute “red flag” laws, said “serious progress” was being made behind closed doors. “I spoke with Senator (Chuck) Schumer this morning. He said he would like to have this bill ready…