serial: sh-sci: Update warning message in sci_request_dma_chan()

The commit below changed a function call from
dma_request_slave_channel_compat() to dma_request_slave_channel(), but
forgot to update the printed failure message.

Fixes: 219fb0c1

 ("serial: sh-sci: Remove the platform data dma slave rx/tx channel IDs")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>