sata_via: magic vt6421 fix for transmission problems w/ WD drives

vt6421 has problems talking to recent WD drives.  It causes a lot of
transmission errors while high bandwidth transfer as reported in the
following bugzilla entry.

  https://bugzilla.kernel.org/show_bug.cgi?id=15173



Joseph Chan provided the following fix.  I don't have any idea what it
does but I can verify the issue is gone with the patch applied.

Signed-off-by: Tejun Heo <tj@kernel.org>
Originally-from: Joseph Chan <JosephChan@via.com.tw>
Reported-by: Jorrit Tijben <sjorrit@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>