spi: rockchip: fix error handling when probe

After failed to request dma tx chain, we need to disable pm_runtime.
Also cleanup error labels for better readability.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>