spi: spi-fsl-dspi: Make bus-num property optional

The SPI bus number is completely optional to Linux, so make the
corresponding device tree property optional as well.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20200305115546.31814-1-s.hauer@pengutronix.deSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-fsl-dspi.txt

@@ -19,11 +19,11 @@ Required properties:
 - pinctrl-0: pin control group to be used for this controller.
 - pinctrl-names: must contain a "default" entry.
 - spi-num-chipselects : the number of the chipselect signals.
-- bus-num : the slave chip chipselect signal number.
 
 Optional property:
 - big-endian: If present the dspi device's registers are implemented
   in big endian mode.
+- bus-num : the slave chip chipselect signal number.
 
 Optional SPI slave node properties:
 - fsl,spi-cs-sck-delay: a delay in nanoseconds between activating chip

drivers/spi/spi-fsl-dspi.c

@@ -1108,7 +1108,7 @@ static int dspi_probe(struct platform_device *pdev)
 	const struct regmap_config *regmap_config;
 	struct fsl_dspi_platform_data *pdata;
 	struct spi_controller *ctlr;
-	int ret, cs_num, bus_num;
+	int ret, cs_num, bus_num = -1;
 	struct fsl_dspi *dspi;
 	struct resource *res;
 	void __iomem *base;
@@ -1145,11 +1145,7 @@ static int dspi_probe(struct platform_device *pdev)
 		}
 		ctlr->num_chipselect = cs_num;
 
-		ret = of_property_read_u32(np, "bus-num", &bus_num);
-		if (ret < 0) {
-			dev_err(&pdev->dev, "can't get bus-num\n");
-			goto out_ctlr_put;
-		}
+		of_property_read_u32(np, "bus-num", &bus_num);
 		ctlr->bus_num = bus_num;
 
 		if (of_property_read_bool(np, "spi-slave"))