spi: rockchip: check requesting dma channel with EPROBE_DEFER

Let's defer probing the driver if the return value of
dma_request_slave_channel is ERR_PTR(-EPROBE_DEFER) instead
of disabling dma capability directly.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>