usbip: change dev_attr_group to constant

The dev_attr_group variable is never changed and it is only passed
to the second parameter of sysfs_create_group() and sysfs_remove_group() functions.
These functions are defined in linux/sysfs.h: the second parameter is a pointer to
const in both cases.
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/usbip/vhci.h

@@ -96,7 +96,7 @@ struct vhci_hcd {
 };
 
 extern struct vhci_hcd *the_controller;
-extern struct attribute_group dev_attr_group;
+extern const struct attribute_group dev_attr_group;
 #define hardware (&the_controller->pdev.dev)
 
 /* vhci_hcd.c */

drivers/staging/usbip/vhci_sysfs.c

@@ -239,6 +239,6 @@ static struct attribute *dev_attrs[] = {
 	NULL,
 };
 
-struct attribute_group dev_attr_group = {
+const struct attribute_group dev_attr_group = {
 	.attrs = dev_attrs,
 };