sata_mv: msi masking fix (v2)

Enable reliable use of Message-Signaled Interrupts (MSI) in sata_mv
by masking further chip interrupts within the main interrupt handler.

Based upon a suggestion by Grant Grundler.
MSI is working reliably in all of my test systems here now.

Signed-off-by: Mark Lord <mlord@pobox.com>
Reviewed-by: Grant Grundler <grundler@google.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>