next-cube: move SCSI 4020/4021 logic from next-pc device to next-scsi device

next-cube: move SCSI 4020/4021 logic from next-pc device to next-scsi device

The SCSI 4020/4021 logic refers to the offset of the SCSI CSRs within the NeXTCube
address space. Due to the previously overlapping memory regions, there were
duplicate MMIO accessors in the next.scr memory region for these registers but
this has now been resolved.

Move the remaining SCSI 4020/4021 logic from the next-pc device to the next-scsi
device, with the exception that the SCSI 4021 register now returns its previous
value like a normal register instead of a hardcoded 0x40 value. This also matches
how the registers are implemented in the Previous emulator.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20241222130012.1013374-9-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>