sata_mv: don't read hc_irq_cause

Remove silly read-modify-write sequences when clearing interrupts
in hc_irq_cause.  This gets rid of unneeded MMIO reads, resulting in
a slight performance boost when switching between EDMA and non-EDMA
modes (eg. for cache flushes).

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