powerpc: Fix logic error in fixup_irqs

powerpc: Fix logic error in fixup_irqs

commit 3cd8519248e9e17d982c6fab0f1a89bca6feb49a upstream.

When SPARSE_IRQ is set, irq_to_desc() can
return NULL. While the code here has a
check for NULL, it's not really correct.
Fix it by separating the check for it.

This fixes CPU hot unplug for me.

Reported-by: Alastair Bridgewater <alastair.bridgewater@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>