staging: gpib: Add missing interface entry point

staging: gpib: Add missing interface entry point

Declaring the driver entry points as static caused a warning
that the serial_poll_status function of the agilent_82350b driver
was unused.

Add the entry point to the corresponding interface structure
initializations where it was missing.

Fixes: 09a4655ee1eb ("staging: gpib: Add HP/Agilent/Keysight 8235xx PCI GPIB driver")
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20250122103859.25499-2-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>