virtio-rng: fix check for period_ms validity

This was reported for this warning:

hw/virtio/virtio-rng.c:150:31: warning: logical not is only applied to
the left hand side of comparison [-Wlogical-not-parentheses]

Reported-by: dcb
Suggested-by: dcb
Bug: https://bugs.launchpad.net/qemu/+bug/1393486
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
