spi: rockchip: covert rsd_nsecs to u32 type

rsd_nsecs is defined as u8 memeber of struct rockchip_spi,
but using of_property_read_u32. That means we take risk of
truncation by type conversion if we pass on big value from
dt.

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