libata: SRST can't be trusted on PMP sil3726

As in sil4726, SRST can't be trusted on sil3726 causing detection
problems under certain configuraitons.  I thought it was from the
Config Disk device but apparently not.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>