mac_via: make SCSI_DATA (DRQ) bit live rather than latched

mac_via: make SCSI_DATA (DRQ) bit live rather than latched

The VIA2 on the Q800 machine is not a separate chip as in older Macs but instead
is integrated into the on-board logic. From analysing the SCSI routines in the
MacOS toolbox ROM (and to a lesser extent NetBSD and Linux) the expectation seems
to be that the SCSI_DATA (DRQ) bit is live on the Q800 and not latched.

Fortunately we can use the recently introduced mos6522 last_irq_levels variable
which tracks the edge-triggered state to return the SCSI_DATA (DRQ) bit live to
the guest OS.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220305150957.5053-11-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>