IDE driver in polling mode

[Alan Cox]

> The only real restriction in libata is in ata_device_add(), which 
> unconditionally registers an irq handler.  Make a few changes there, add 
> a few flags, and the existing polling code will kick in.

Obvious thing to do would be to do this if the IRQ number passed in is 0
rather than aborting as we do now. In theory ISAPnP devices may be
IRQless too.

Alan