Always add 'config' emptyDir volume to MPS daemonset

[cdesiniotis]

This was missed in #1070

The following error is resolved with this PR:

$ kubectl logs -n gpu-operator nvidia-device-plugin-mps-control-daemon-j57tn -c config-manager-init
W1028 20:45:36.571294     120 client_config.go:659] Neither --kubeconfig nor --master was specified.  Using the inClusterConfig.  This might not work.
I1028 20:45:36.571587     120 main.go:246] Waiting for change to 'nvidia.com/device-plugin.config' label
I1028 20:45:36.591680     120 main.go:248] Label change detected: nvidia.com/device-plugin.config=
I1028 20:45:36.591906     120 main.go:360] No value set. Selecting default name: mps
I1028 20:45:36.591922     120 main.go:304] Updating to config: mps
E1028 20:45:36.592027     120 main.go:208] error creating symlink: symlink /available-configs/mps /config/config.yaml: no such file or directory