ppc64 has an optimisation where it loops on get_irq until there are no more
interrupts to be handled.  Mark Hack notes that this optimisation hardly
ever hits and costs us a potentially expensive extra read of an interrupt
register every interrupt.

Also make do_IRQ void, the callers never use the return value.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>