driver-core: return EINVAL error instead of BUG_ON()

I triggerd the BUG_ON() in driver_register() when booting a domU Xen
domain. Since there was no contextual information logged, I needed to
attach kgdb to determine the culprit (the wmi-bmof driver in my
case). The BUG_ON() was added in commit f48f3feb ("driver-core: do
not register a driver with bus_type not registered").

Instead of running into a BUG_ON() we print an error message
identifying the, likely faulty, driver but continue booting.
Signed-off-by: Florian Schmaus <flo@geekplace.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/driver.c

@@ -148,7 +148,11 @@ int driver_register(struct device_driver *drv)
 	int ret;
 	struct device_driver *other;
 
-	BUG_ON(!drv->bus->p);
+	if (!drv->bus->p) {
+		pr_err("Driver '%s' was unable to register with bus_type '%s' because the bus was not initialized.\n",
+			   drv->name, drv->bus->name);
+		return -EINVAL;
+	}
 
 	if ((drv->bus->probe && drv->probe) ||
 	    (drv->bus->remove && drv->remove) ||