hwmon: (acpi_power_meter) clean up freeing code

This code works okay but Smatch flagged it as a double free.  I've
changed three things to make it more clear.  1)  Remove the call to
free_capabilities() in acpi_power_meter_add().  This call is a no-op
because the capabilities have not been allocated yet.  2)  Set "*str" to
NULL in free_capabilities() so that way the function can be called twice
in a row without leading to a double free.  3)  Call free_capabilities()
in read_capabilities() instead of open coding the free.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201007075148.GB2529578@mwandaSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/acpi_power_meter.c

@@ -725,8 +725,10 @@ static void free_capabilities(struct acpi_power_meter_resource *resource)
 	int i;
 
 	str = &resource->model_number;
-	for (i = 0; i < 3; i++, str++)
+	for (i = 0; i < 3; i++, str++) {
 		kfree(*str);
+		*str = NULL;
+	}
 }
 
 static int read_capabilities(struct acpi_power_meter_resource *resource)
@@ -801,9 +803,7 @@ static int read_capabilities(struct acpi_power_meter_resource *resource)
 	dev_info(&resource->acpi_dev->dev, "Found ACPI power meter.\n");
 	goto end;
 error:
-	str = &resource->model_number;
-	for (i = 0; i < 3; i++, str++)
-		kfree(*str);
+	free_capabilities(resource);
 end:
 	kfree(buffer.pointer);
 	return res;
@@ -874,7 +874,6 @@ static int acpi_power_meter_add(struct acpi_device *device)
 	strcpy(acpi_device_class(device), ACPI_POWER_METER_CLASS);
 	device->driver_data = resource;
 
-	free_capabilities(resource);
 	res = read_capabilities(resource);
 	if (res)
 		goto exit_free;