sata_mv: fix 8-port timeouts on 508x/6081 chips

Fix a longstanding bug for the 8-port Marvell Sata controllers (508x/6081),
where accesses to the upper 4 ports would cause lost-interrupts / timeouts
for the lower 4-ports.  With this patch, the 6081 boards should finally be
reliable enough for mainstream use with Linux.

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