[POWERPC] Compare irq numbers with NO_IRQ not IRQ_NONE

There is a thinko in the irq code, it uses IRQ_NONE to indicate no irq,
whereas it should be using NO_IRQ.  IRQ_NONE is returned from irq
handlers to say "not handled".

As it happens they currently have the same value (0), so this is just for
future proof-ness.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>