spi: rockchip: remove xfer_completion from rockchip_spi

xfer_completion isn't been used anywhere, so it can be
removed.
Signed-off-by: NShawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi-rockchip.c

@@ -192,8 +192,6 @@ struct rockchip_spi {
 	/* protect state */
 	spinlock_t lock;
 
-	struct completion xfer_completion;
-
 	u32 use_dma;
 	struct sg_table tx_sg;
 	struct sg_table rx_sg;