mem-cache: Fix update of useful prefetches

mem-cache: Fix update of useful prefetches

The probe notification must be parsed on every hit, even if
the prefetcher is set not to generate prefetches on accesses.
This fixes the calculation of useful prefetches.

Change-Id: Iff298f7bea11013f411f4ba39dba705fd81a0cd4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38177
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>