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

Don't blindly enable port IRQs at host init time.
Instead, enable only the bits that we want,
which in this case is simply the PCI_ERR bit.

The per-port bits can wait until the ports are reset/probed for devices.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>