ASoC: max98373: check for device node before parsing

Below Oops is caused in a system which uses ACPI instead of device node:

of_get_named_gpiod_flags: can't parse 'maxim,reset-gpio' property of node '(null)[0]'
BUG: kernel NULL pointer dereference, address: 0000000000000010

This patch avoids NULL pointer deferencing by adding a check before parsing
and initializes to make reset-gpio pin as invalid.
Signed-off-by: Sathyanarayana Nujella <sathyanarayana.nujella@intel.com>
Signed-off-by: Jairaj Arava <jairaj.arava@intel.com>
Link: https://lore.kernel.org/r/1569702150-11976-1-git-send-email-sathyanarayana.nujella@intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/max98373.c

@@ -901,16 +901,20 @@ static void max98373_slot_config(struct i2c_client *i2c,
 		max98373->i_slot = value & 0xF;
 	else
 		max98373->i_slot = 1;
-
-	max98373->reset_gpio = of_get_named_gpio(dev->of_node,
+	if (dev->of_node) {
+		max98373->reset_gpio = of_get_named_gpio(dev->of_node,
 						"maxim,reset-gpio", 0);
-	if (!gpio_is_valid(max98373->reset_gpio)) {
-		dev_err(dev, "Looking up %s property in node %s failed %d\n",
-			"maxim,reset-gpio", dev->of_node->full_name,
-			max98373->reset_gpio);
+		if (!gpio_is_valid(max98373->reset_gpio)) {
+			dev_err(dev, "Looking up %s property in node %s failed %d\n",
+				"maxim,reset-gpio", dev->of_node->full_name,
+				max98373->reset_gpio);
+		} else {
+			dev_dbg(dev, "maxim,reset-gpio=%d",
+				max98373->reset_gpio);
+		}
 	} else {
-		dev_dbg(dev, "maxim,reset-gpio=%d",
-			max98373->reset_gpio);
+		/* this makes reset_gpio as invalid */
+		max98373->reset_gpio = -1;
 	}
 
 	if (!device_property_read_u32(dev, "maxim,spkfb-slot-no", &value))