atp870u: Fix incorrect writeb_io access to register 0x3a

The ioport region is 0x20 bytes long so accessing 0x3a register using
writeb_io is incorrect. Use writeb_base instead.
There's no change in behavior as 870 chips have ioport = baseport.

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>