spi: rockchip: avoid objtool warning

Building this file with clang leads to a an unreachable code path
causing a warning from objtool:

drivers/spi/spi-rockchip.o: warning: objtool: rockchip_spi_transfer_one()+0x2e0: sibling call from callable instruction with modified stack frame

Change the unreachable() into an error return that can be
handled if it ever happens, rather than silently crashing
the kernel.

Fixes: 65498c6a ("spi: rockchip: support 4bit words")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20210226140109.3477093-1-arnd@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-rockchip.c

@@ -476,7 +476,7 @@ static int rockchip_spi_prepare_dma(struct rockchip_spi *rs,
 	return 1;
 }
 
-static void rockchip_spi_config(struct rockchip_spi *rs,
+static int rockchip_spi_config(struct rockchip_spi *rs,
 		struct spi_device *spi, struct spi_transfer *xfer,
 		bool use_dma, bool slave_mode)
 {
@@ -521,7 +521,9 @@ static void rockchip_spi_config(struct rockchip_spi *rs,
 		 * ctlr->bits_per_word_mask, so this shouldn't
 		 * happen
 		 */
-		unreachable();
+		dev_err(rs->dev, "unknown bits per word: %d\n",
+			xfer->bits_per_word);
+		return -EINVAL;
 	}
 
 	if (use_dma) {
@@ -554,6 +556,8 @@ static void rockchip_spi_config(struct rockchip_spi *rs,
 	 */
 	writel_relaxed(2 * DIV_ROUND_UP(rs->freq, 2 * xfer->speed_hz),
 			rs->regs + ROCKCHIP_SPI_BAUDR);
+
+	return 0;
 }
 
 static size_t rockchip_spi_max_transfer_size(struct spi_device *spi)
@@ -577,6 +581,7 @@ static int rockchip_spi_transfer_one(
 		struct spi_transfer *xfer)
 {
 	struct rockchip_spi *rs = spi_controller_get_devdata(ctlr);
+	int ret;
 	bool use_dma;
 
 	WARN_ON(readl_relaxed(rs->regs + ROCKCHIP_SPI_SSIENR) &&
@@ -596,7 +601,9 @@ static int rockchip_spi_transfer_one(
 
 	use_dma = ctlr->can_dma ? ctlr->can_dma(ctlr, spi, xfer) : false;
 
-	rockchip_spi_config(rs, spi, xfer, use_dma, ctlr->slave);
+	ret = rockchip_spi_config(rs, spi, xfer, use_dma, ctlr->slave);
+	if (ret)
+		return ret;
 
 	if (use_dma)
 		return rockchip_spi_prepare_dma(rs, ctlr, xfer);