spi: rockchip: fix probe deferral handling

Use dma_request_chan instead of dma_request_slave_channel,
in this case we can check EPROBE_DEFER without static
warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>