A Randomized, Open-Label, Cross Over Trial, Assessing Ease of use, Correctness of use and Patient Preference of Fluticasone Propionate Inhalation Powder (100mcg BID) via the DISKUS™ versus Fluticasone Propionate Aerosol via the Metered Dose Inhaler (88mcg BID) in Subjects at Least 12 Years of Age with Persistent Asthma Currently Receiving Short Acting Beta Agonists Alone., Trial ID FPD40016 | GSK