serial: sh-sci: Postpone DMA release when falling back to PIO

When the sh-sci driver detects an issue with DMA during operation, it
falls backs to PIO, and releases all DMA resources.

As releasing DMA resources immediately has no advantages, but
complicates the code, and is susceptible to races, it is better to
postpone this to port shutdown.

This allows to remove the locking from sci_rx_dma_release() and
sci_tx_dma_release(), but requires keeping a copy of the DMA channel
pointers for release during port shutdown.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>