isci: rely on irq core for intx multiplexing, and silence screaming intx

Remove the extra logic to poll each controller for interrupts, that's
the core's job for shared interrupts.

While testing noticed that a number of interrupts fire while waiting for
the completion tasklet to run, so added an irq-ack.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>