Possible bug in IRQ handling in pata_of_platform / pata_platform

[Steven A. Falco]

I think there is a bug in the communications between pata_of_platform
and pata_platform.  I will refer to the master branch of the DENX git
tree, which is roughly v2.6.26.1 at this time.  I am using a Sequoia
board with a PPC440EPx.

In pata_of_platform, we have:

    ret = of_irq_to_resource(dn, 0, &irq_res);
    if (ret == NO_IRQ)
        irq_res.start = irq_res.end = -1;

so if there is no interrupt defined, then start and end are -1. 
However, __pata_platform_probe has:

    if (irq_res && irq_res->start > 0) {
        irq = irq_res->start;
        irq_flags = irq_res->flags;
    }

You might think that the (irq_res->start > 0) test will fail, as it
should in this no-irq case.  But, start is a u64, so the -1 actually
looks like a large positive number in the comparison.  So,
__pata_platform_probe attempts to use an interrupt when there isn't one.

I think the fix would be to change __pata_platform_probe to:

    if (irq_res && irq_res->start != -1) {

but that might have other unintended consequences, so I'll defer to
whomever knows more about the intent of this code.

    Steve