sata_mv: cache main_irq_mask register in hpriv

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

Keep a cached copy of the main_irq_mask so that we don't have
to stall the CPU to read it on every pass through mv_interrupt.

This significantly speeds up interrupt handling, both for sata_mv,
and for any other driver/device sharing the same PCI IRQ line.

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