mem-cache: Workaround for setWhenReady assertion

mem-cache: Workaround for setWhenReady assertion

Change 174da8e2da6a896d2e97bc264f9c827a0f4c35ac added an assert that
is not satisfiable with current implementation, breaking some
regression tests.

Change-Id: Ibafaf0c51906384364f0b2a4b931f8ec6126d858
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/14955
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>