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

Partially fix a reported bug whereby we sometimes miss seeing drives on
a port-multiplier, as reported by Gwendal Grignou <gwendal@google.com>.

The problem was that we were receiving unexpected interrupts
during EH from POLLed commands while accessing port-multiplier registers.
These unexpected interrupts can be prevented by masking the DONE_IRQ bit
for the port whenever not operating in EDMA mode.

Also fix port_stop() to mask all port interrupts.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>