atp870u: Introduce HW access wrappers

Introduce *_read? and *_write? wrappers to improve code readability.
Also make sure that baseport is always initialized, not only for ATP880.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>