driver model: Don't reset dev->driver until after we call dev->driver->remove.

This fixes an Oops in the USB code that was using ->driver, both for checking what driver to
use and calling into it. 

From Greg KH.

drivers/base/core.c

@@ -97,18 +97,13 @@ static int device_attach(struct device * dev)
 
 static void device_detach(struct device * dev)
 {
-	struct device_driver * drv; 
+	struct device_driver * drv = dev->driver;
 
-	if (dev->driver) {
+	if (drv) {
 		devclass_remove_device(dev);
-		spin_lock(&device_lock);
-		drv = dev->driver;
-		dev->driver = NULL;
-		spin_unlock(&device_lock);
-
-		/* detach from driver */
 		if (drv && drv->remove)
 			drv->remove(dev);
+		dev->driver = NULL;
 	}
 }