pata_sil680: PIO1 taskfile transfers overclocking fix (repost)

Fix PIO mode 1 overclocked taskfile transfers -- probably a typo carried over
from drivers/ide/pci/siimage.c where I've found it by documentation check...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>