sata_mv: consolidate main_irq_mask updates

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

Consolidate all updates of the host main_irq_mask register
into a single function.  This simplifies maintenance,
and also prepares the way for caching it (later).

No functionality changes in this update.

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