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

drivers/spi/spi-rockchip.c

@@ -173,7 +173,7 @@ struct rockchip_spi {
 	u8 tmode;
 	u8 bpw;
 	u8 n_bytes;
-	u8 rsd_nsecs;
+	u32 rsd_nsecs;
 	unsigned len;
 	u32 speed;