[S390] css_bus_type: make it static

Make css_bus_type static. Subchannel drivers have to
use css_driver_register.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c
index 51eaed0..c47b25f 100644
--- a/drivers/s390/cio/css.c
+++ b/drivers/s390/cio/css.c
@@ -35,6 +35,7 @@
 int max_ssid;
 
 struct channel_subsystem *channel_subsystems[__MAX_CSSID + 1];
+static struct bus_type css_bus_type;
 
 int
 for_each_subchannel(int(*fn)(struct subchannel_id, void *), void *data)
@@ -1214,7 +1215,7 @@
 	.restore = css_pm_restore,
 };
 
-struct bus_type css_bus_type = {
+static struct bus_type css_bus_type = {
 	.name     = "css",
 	.match    = css_bus_match,
 	.probe    = css_probe,
@@ -1251,4 +1252,3 @@
 EXPORT_SYMBOL_GPL(css_driver_unregister);
 
 MODULE_LICENSE("GPL");
-EXPORT_SYMBOL(css_bus_type);