staging: gpib: Fix Oops after disconnect in agilent usb

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.

Fixes: fbae7090f30c ("staging: gpib: Update messaging and usb_device refs in agilent_usb")
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250222204515.5104-1-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>