sata_mv: disregard masked irqs

Part four of simplifying/fixing handling of the main_irq_mask register
to resolve unexpected interrupt issues observed in 2.6.26-rc*.

Ignore masked IRQs in mv_interrupt().
This prevents "unexpected device interrupt while idle" messages.

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