spi: Fix potential uninitialized variable in probe()

If the device tree is messed up, then potentially the "protocol" string
could potentially be uninitialized.  The property is supposed to default
to "motorola" so if the of_property_read_string() function returns
-EINVAL then default to "motorola".

Fixes: 059f545 ("spi: add support for microchip "soft" spi controller")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Link: https://patch.msgid.link/aSQPkfkiJ0w-FJMW@stanley.mountain
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Dan Carpenter

drivers/spi/spi-microchip-core-spi.c

@@ -295,10 +295,10 @@ static int mchp_corespi_transfer_one(struct spi_controller *host,
 
 static int mchp_corespi_probe(struct platform_device *pdev)
 {
+	const char *protocol = "motorola";
 	struct spi_controller *host;
 	struct mchp_corespi *spi;
 	struct resource *res;
-	const char *protocol;
 	u32 num_cs, mode, frame_size;
 	bool assert_ssel;
 	int ret = 0;
@@ -320,6 +320,8 @@ static int mchp_corespi_probe(struct platform_device *pdev)
 	 */
 	ret = of_property_read_string(pdev->dev.of_node, "microchip,protocol-configuration",
 				      &protocol);
+	if (ret && ret != -EINVAL)
+		return dev_err_probe(&pdev->dev, ret, "Error reading protocol-configuration\n");
 	if (strcmp(protocol, "motorola") != 0)
 		return dev_err_probe(&pdev->dev, -EINVAL,
 				     "CoreSPI: protocol '%s' not supported by this driver\n",