st-hal: Prevent double second stage prepare/stop

Second stage prepare/stop functions should only be called as
part of the client load/unload sequences. Currently, they are
also erroneously called internally within the SSR state function.
So if client load or unload occurs during SSR, these functions can
be called twice, which causes failures.

Change-Id: I22c1878db37dfebf5464020bc07651bf0b239600
1 file changed