The new “Rush Radio” station has taken off, nearly quadrupling its audience since flipping from a Spanish music station to a conservative news talker last month. WXKS-AM (1200) - which snagged Rush Limbaugh from right-wing rival WRKO-AM (680) - fetched a 1.1 share of all Boston listeners older than 6 during the month of March, according to Arbitron ratings released yesterday.