ncr5380: Use runtime register mapping

Convert compile-time C400_ register mapping to runtime mapping.
This removes the weird negative register offsets and allows adding
additional mappings.

While at it, convert read/write loops into insb/outsb.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>