cpu: Fix a = that was supposed to be a == in an assert.

cpu: Fix a = that was supposed to be a == in an assert.

The KVM CPU has a _status field which is checked by an assert, but
rather than checking it with an ==, it accidentally used a =.

Change-Id: Ic1970d232786af6666c4ec2719c70f3f1509277c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29405
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>