sata_mv: async notify for genIIe only

Now that we handle the FIS_IRQ_CAUSE register correctly,
we can also now handle SATA asynchronous notification events.

So enable them, but only for the more modern GenIIe chips.
(older chips have unaddressed errata issues related to this).

This fixes hot plug/unplug for port-muliplier ports.

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