[libata] pata_rb532_cf: fix and rename register definitions

The original standalone driver uses a custom address for the error
register. Use it in pata_rb532_cf, too.

Rename two register definitions:
- The address offset 0x0800 in fact is the ATA base, not ATA command
  address.
- The offset 0x0C00 is not a regular ATA data address, but a buffered one
  allowing 4-byte IO.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>