mem-garnet: Initialize unused Credit members

mem-garnet: Initialize unused Credit members

The Credit class doesn't initialize a number of its unused base class
fields.  This leads to non-determanistic traces when printing flits that
are Credits.  This patch initializes all unused fields to 0.

Change-Id: Ib73c652c71a10be57b24c0d6e1ac22eafa421e11
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32598
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>