usbip: move usbip kernel code out of staging

At this point, USB/IP kernel code is fully functional
and can be moved out of staging.

Signed-off-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/usb/Makefile b/drivers/usb/Makefile
index 3cba892..d7be717 100644
--- a/drivers/usb/Makefile
+++ b/drivers/usb/Makefile
@@ -60,3 +60,5 @@
 obj-$(CONFIG_USB_GADGET)	+= gadget/
 
 obj-$(CONFIG_USB_COMMON)	+= common/
+
+obj-$(CONFIG_USBIP_CORE)	+= usbip/