Fix 8259 IRQ priority resolver.

[neel]

Summary

Fix 8259 IRQ priority resolver.
Initialize the 8259 such that IRQ7 is the lowest priority.

Test Plan

I wrote a small C program to dump the priorities using the current method
and the new method. It is easy to see the difference in behavior using this.

oldprio(7) 0 1 2 3 4 5 6 7
newprio(7) 0 1 2 3 4 5 6 7

oldprio(6) 1 2 3 4 5 6 7 0
newprio(6) 7 0 1 2 3 4 5 6

oldprio(5) 2 3 4 5 6 7 0 1
newprio(5) 6 7 0 1 2 3 4 5

oldprio(4) 3 4 5 6 7 0 1 2
newprio(4) 5 6 7 0 1 2 3 4

oldprio(3) 4 5 6 7 0 1 2 3
newprio(3) 4 5 6 7 0 1 2 3

oldprio(2) 5 6 7 0 1 2 3 4
newprio(2) 3 4 5 6 7 0 1 2

oldprio(1) 6 7 0 1 2 3 4 5
newprio(1) 2 3 4 5 6 7 0 1

oldprio(0) 7 0 1 2 3 4 5 6
newprio(0) 1 2 3 4 5 6 7 0

[neel]

The C program mentioned in the Test Plan:

#include <stdio.h>

static void
dump_old_priorities(int prio)
{
	int i, pin;

	for (i = 0; i < 8; i++) {
		pin = (i + 7 - prio) & 0x7;
		printf(" %d", pin);
	}
}

#define ATPIC_PIN_FOREACH(pinvar, tmpvar)                        	\
        for (tmpvar = 0, pinvar = (prio + 1) & 0x7;           		\
            tmpvar < 8;                                                 \
            tmpvar++, pinvar = (pinvar + 1) & 0x7)

static void
dump_new_priorities(int prio)
{
	int pin, tmp;

	ATPIC_PIN_FOREACH(pin, tmp) {
		printf(" %d", pin);
	}
}

int
main(int argc, char *argv[])
{
	int prio;

	for (prio = 7; prio >= 0; prio--) {
		printf("oldprio(%d)\t", prio);
		dump_old_priorities(prio);
		printf("\n");
		printf("newprio(%d)\t", prio);
		dump_new_priorities(prio);
		printf("\n\n");
	}
}

[tychon]

I had written a similar test-stub when I wrote the code originally. Obviously, I misinterpreted the results :-(

Regardless, this change looks good!