A Randomized, Double-Blind, Placebo-Controlled, Parallel Group, Multicenter Study to Evaluate the Efficacy and Safety of GW685698X 200mcg Twice Daily, GW685698X 200mcg and 400mcg Once Daily in the Morning, and GW685698X 200mcg and 400mcg Once Daily in the Evening Compared with Placebo for 8 Weeks in Adolescent and Adult Subjects (12 Years of Age and Older) with Persistent Asthma.