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: NAlastair Bridgewater <alastair.bridgewater@gmail.com>
Cc: stable@kernel.org [2.6.32+]
Signed-off-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>