mem-cache: Remove extra numSets zero check.

mem-cache: Remove extra numSets zero check.

numSets is unsigned, so it cannot be lower than 0. Besides, isPowerOf2(0)
is false by definition (and implemmentation*), so there is no need for the
double check.

* As presented in base/intmath.hh

Change-Id: I3f6296694a937434feddc7ed21f11c2a6fdfc5a9
Reviewed-on: https://gem5-review.googlesource.com/7901
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>