sata_mv: fix irq mask races

Prevent racing on the main interrupt mask during port_start and port_stop.
Otherwise, we end up with IRQs masked on inactive ports,
and hotplug insertions then get missed later on.

Found while debugging (out of tree) target mode operations,
but the bug is present and impacting mainline as well.

This patch should also be considered for -stable.

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