vdpa: fix comment of vdpa_register_device()

The function should be called only after a success calling for
vdpa_alloc_device().
Reported-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200413093738.28467-1-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa.c

@@ -116,7 +116,7 @@ EXPORT_SYMBOL_GPL(__vdpa_alloc_device);
 
 /**
  * vdpa_register_device - register a vDPA device
- * Callers must have a succeed call of vdpa_init_device() before.
+ * Callers must have a succeed call of vdpa_alloc_device() before.
  * @vdev: the vdpa device to be registered to vDPA bus
  *
  * Returns an error when fail to add to vDPA bus