mfd: core: Fix platform-device id generation

Make sure to always honour multi-function devices registered with
PLATFORM_DEVID_NONE (-1) or PLATFORM_DEVID_AUTO (-2) as id base. In this
case it does not make sense to append the cell id to the mfd-id base and
potentially change the requested behaviour.

Specifically this will allow multi-function devices to be registered
with PLATFORM_DEVID_AUTO while still having non-zero cell ids.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/mfd-core.c

@@ -125,9 +125,15 @@ static int mfd_add_device(struct device *parent, int id,
 	struct platform_device *pdev;
 	struct device_node *np = NULL;
 	int ret = -ENOMEM;
+	int platform_id;
 	int r;
 
-	pdev = platform_device_alloc(cell->name, id + cell->id);
+	if (id < 0)
+		platform_id = id;
+	else
+		platform_id = id + cell->id;
+
+	pdev = platform_device_alloc(cell->name, platform_id);
 	if (!pdev)
 		goto fail_alloc;