serial: sh-sci: Use correct device for DMA mapping with IOMMU

To function correctly in the presence of an IOMMU, the DMA buffers must
be managed using the DMA channel's device instead of the platform
device's device.

Make sure to free the DMA memory before releasing the channel, not
after.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>