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@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>