dmx3191d: Use NO_IRQ

Testing shows that the Domex 3191D card never asserts its IRQ. Hence it is
non-functional with Linux (worse, the EH bugs in the core driver are fatal
but that's a problem for another patch). Perhaps the DT-536 chip needs
special setup? I can't find documentation for it. The NetBSD driver uses
polling apparently because of this issue.

Set host->irq = NO_IRQ so the core driver will prevent targets from
disconnecting. Don't request host->irq.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>