serial: sh-sci: Extract sci_dma_rx_chan_invalidate()

The cookies and channel pointer for the DMA receive channel are
invalidated in two places, and one more is planned.
Extract this functionality in a common helper.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>