[PATCH] sata_via: use old SCR access pattern on vt6420

[PATCH] sata_via: use old SCR access pattern on vt6420

vt6420 has super-fragile SCR registers which can hang the whole
machine if accessed with the wrong timings.  This patch makes sata_via
use SCR registers only during probing and with the same timings as
before (pre new EH), which is proven to work.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>