scsi: virtio_scsi: Demote seemingly unintentional kerneldoc header

scsi: virtio_scsi: Demote seemingly unintentional kerneldoc header

This is the only use of kerneldoc in the sourcefile and no descriptions are
provided.

Fixes the following W=1 kernel build warning(s):

 drivers/scsi/virtio_scsi.c:109: warning: Function parameter or member 'vscsi' not described in 'virtscsi_complete_cmd'
 drivers/scsi/virtio_scsi.c:109: warning: Function parameter or member 'buf' not described in 'virtscsi_complete_cmd'

Link: https://lore.kernel.org/r/20200713080001.128044-13-lee.jones@linaro.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>