spi: rockchip: Fix error in getting num-cs property

Get num-cs u32 from dts of_node property rather than u16.

Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Link: https://lore.kernel.org/r/20220216014028.8123-2-jon.lin@rock-chips.com


Signed-off-by: Mark Brown <broonie@kernel.org>