serial: sh-sci: Switch to generic DMA residue handling

Convert the SCI driver from the SHDMAE-specific partial DMA transfer
handling to the generic dmaengine residual data framework.

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