mem-cache: Fix setting prefetch bit

mem-cache: Fix setting prefetch bit

Commit https://gem5-review.googlesource.com/c/public/gem5/+/35699
had a copy-paste error: when setting the prefetch bit it must
become true.

Change-Id: Ib0abc5141dd65d3c739dc01948a72eb5451884e8
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38176
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>