serial: sh-sci: Make RX/TX DMA function names consistent

Most RX/TX-specific DMA functions are prefixed with "sci_dma_[rt]x_".
Rename the exceptions to increase consistency.

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>