SiI3124 might lose completion interrupt if completion interrupt occurs
shortly after SLOT_STAT register is read for the previous completion
interrupt if it is operating in PCI-X mode.

This currently doesn't trigger as libata never queues more than one
command, but it will with NCQ changes.  This patch implements the
workaround - turning on WoC and explicitly clearing interrupt.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>