sata_mv fix mv_host_intr bug for hc_irq_cause

Remove the unwanted reads of hc_irq_cause from mv_host_intr(),
thereby removing a bug whereby we were not always reading it when needed..

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>