[PATCH] powerpc: Fix PowerMac IRQ handling bug

The port to genirq & the new powerpc interrupt model in 2.6.18 introduced a
bug in the legacy PowerMac PIC code (used on older machines) because of a
typo potentially causing hangs due to interrupt storms.  This fixes it,
along with a performance issue causing us to do spurrious retriggers after
masking an interrupt.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>