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.