usb: core: Don't force USB generic_subclass drivers to define probe()

There's no real reason that subclassed USB drivers _need_ to define
probe() since they might want to subclass for some other reason. Make
it optional to define probe() if we're a generic_subclass.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20231201102946.v2.1.I7ea0dd55ee2acdb48b0e6d28c1a704ab2c29206f@changeidSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/driver.c

@@ -290,7 +290,10 @@ static int usb_probe_device(struct device *dev)
 	 * specialised device drivers prior to setting the
 	 * use_generic_driver bit.
 	 */
-	error = udriver->probe(udev);
+	if (udriver->probe)
+		error = udriver->probe(udev);
+	else if (!udriver->generic_subclass)
+		error = -EINVAL;
 	if (error == -ENODEV && udriver != &usb_generic_driver &&
 	    (udriver->id_table || udriver->match)) {
 		udev->use_generic_driver = 1;