virtio-mem: drop unnecessary initialization

rc is initialized to -ENIVAL but that's never used. Drop it.

Fixes: 5f1f79bb

 ("virtio-mem: Paravirtualized memory hotplug")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>