systemc: Use the stage of simulation and not port size in add_trace.

systemc: Use the stage of simulation and not port size in add_trace.

The assumption was that a port wouldn't have any interfaces until
after elaboration, and that if it would be traced, it would have
interfaces. Checking if the simulation has started (and hence
elaboration has finished) is a more accurate and direct way to check
the same thing.

Change-Id: I7fe9ecea469997d1a257dd3e4a0db31889aff722
Reviewed-on: https://gem5-review.googlesource.com/c/13195
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>