scsi: atari_scsi: Reset DMA during bus reset only under ST-DMA lock

The atari_scsi driver should not access Falcon DMA chip registers unless
it has acquired exclusive access to that chip. If the driver doesn't
have exclusive access then there's no need for a DMA reset as there are
no scsi commands in progress.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>