spi/rockchip: fix bug that cause the failure to read data in DMA mode

In my test on RK3288-pinky board, if spi is enabled, it will begin to
read data from slave regardless of whether the DMA is ready. So we
need prepare DMA before spi is enable.

Signed-off-by: Addy Ke <addy.ke@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org