refine definition/use of ep_max_data_size and ep_max_size

This is very minor but it appears to be that ep_max_data_size and ep_max_size represents basically the same value. However, ep_max_size has the value change listener for allowing change its value dynamically. Either remove one of stat if found to be redundant or re-define each stat and its use.

Also, it is probably a good idea to revisit our stats.org and find if there are any more redundant & intermingled case like this for another stats.