mem-garnet: Fix default value of network bridge

mem-garnet: Fix default value of network bridge

Initializing the network bridge with NULL causes it to have
an class error when instatiating a link. The bridge is only
needed whne either a CDC or SerDes is enabled. This is handled
later during construction of the GarnetLink.

Change-Id: If19a21a6d9bf49449b9c390467d08d3422ae991a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34257
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>