[libata] ahci: SB600 workaround is suspect... play it safe for now

At least one report claims that a878539e
failed to solve lockups, whereas the old limit-to-32-bit trick worked.

Restore the 32-bit limit, but also leave the 255-sector limit in place,
because we know that's needed as well.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>