staging: gpib: Fix NULL pointer dereference in detach

staging: gpib: Fix NULL pointer dereference in detach

When the detach function is called after a failed attach
the usb_dev initialization can cause a NULL pointer
dereference. This happens when the usb device is not found
in the attach procedure.

Remove the usb_dev variable and initialization and change the dev
in the dev_info message from the usb_dev to the gpib_dev.

Fixes: fbae7090f30c ("staging: gpib: Update messaging and usb_device refs in agilent_usb")
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250118145046.12181-2-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>