scsi: sun_esp: fix device reference leaks

Make sure to drop the reference to the dma device taken by
of_find_device_by_node() on probe errors and on driver unbind.

Fixes: 334ae614 ("sparc: Kill SBUS DVMA layer.")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>