nubus: generate bus error when attempting to access empty slots

According to "Designing Cards and Drivers for the Macintosh Family" any attempt
to access an unimplemented address location on Nubus generates a bus error. MacOS
uses a custom bus error handler to detect empty Nubus slots, and with the current
implementation assumes that all slots are occupied as the Nubus transactions
never fail.

Switch nubus_slot_ops and nubus_super_slot_ops over to use {read,write}_with_attrs
and hard-code them to return MEMTX_DECODE_ERROR so that unoccupied Nubus slots
will generate the expected bus error.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210924073808.1041-9-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>