fix "set but not used" in the tws driver

[rew]

Summary

With __diagused, these warnings were still emitted since INVARIANTS is
defined but TWS_DEBUG is not.

Not sure if it's better to go with __tws_debug or something like:

#ifdef TWS_DEBUG
        u_int32_t reg;
#endif

[imp]

__tws_debug is a little shorter, but it's also a bit odd... Since you already have the right place to drop it into tws_services.h, thats likely pushes things in favor of that instead of #ifdef TWS_DEBUG or similar.