Currently the target selection for PDMA is done after the SCSI command has been
delivered which is not correct. Perform target selection as part of the initial
get_cmd() call when the command is submitted: if no target is present, don't
raise DRQ.

If the target is present then switch to the command phase since the MacOS toolbox
ROM checks for this before attempting to submit the SCSI command.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-28-mark.cave-ayland@ilande.co.uk>