audio preprocessing: fix multiple enable problem

Do not set the effect state back to configured when a configuration
command is received while the effect is enabled. Instead just check that
the new config is the same as current config.

It is normal to receive a config command from the framework while enabled
if a new effect is added on the same session.

Change-Id: I93aa38bf60a3d7cc7729934e87ddd69bf1112cd6