staging: gpib: Fix Oops after disconnect in ni_usb
In the Linux kernel, the following vulnerability has been resolved:

staging: gpib: Fix Oops after disconnect in ni_usb

If the 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 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.