systemc: Don't schedule the ready event unnecessarily.

systemc: Don't schedule the ready event unnecessarily.

If we're already going to process the thing we'd be scheduling it to
process, just let the existing invocation get to it.

Change-Id: Ifeebc80903065567fc0eed02beefec6156b22ff7
Reviewed-on: https://gem5-review.googlesource.com/c/12964
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>