staging/usbip: Mark local functions as static (fix sparse warnings)

sparse complains about these functions:
usbip/stub_dev.c:529:5: warning: symbol 'stub_pre_reset' was not declared. Should it be static?
usbip/stub_dev.c:535:5: warning: symbol 'stub_post_reset' was not declared. Should it be static?

-> add static keyword to silence the warning and make sparse happy.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/usbip/stub_dev.c

@@ -526,13 +526,13 @@ static void stub_disconnect(struct usb_interface *interface)
  * when the device is being reset
  */
 
-int stub_pre_reset(struct usb_interface *interface)
+static int stub_pre_reset(struct usb_interface *interface)
 {
 	dev_dbg(&interface->dev, "pre_reset\n");
 	return 0;
 }
 
-int stub_post_reset(struct usb_interface *interface)
+static int stub_post_reset(struct usb_interface *interface)
 {
 	dev_dbg(&interface->dev, "post_reset\n");
 	return 0;