staging: gpib: Fix Oops after disconnect in agilent usb

In the Linux kernel, the following vulnerability has been resolved: staging: gpib: Fix Oops after disconnect in agilent usb If the agilent usb dongle is disconnected subsequent calls to the driver cause a NULL dereference Oops as the bus_interface is set to NULL on disconnect. This problem was introduced by setting usb_dev from the bus_interface for dev_xxx messages. Previously bus_interface was checked for NULL only in the functions directly calling usb_fill_bulk_urb or usb_control_msg. Check for valid bus_interface on all interface entry points and return -ENODEV if it is NULL.