powerpc: eeh: Fixup the brown paperbag fallout of the "cleanup"

Commit b8a9a11b

 (powerpc: eeh: Kill another abuse of irq_desc) is
missing some brackets .....

It's not a good idea to write patches in grumpy mode and then forget
to at least compile test them or rely on the few eyeballs discussing
that patch to spot it.....

Reported-by:  <fengguang.wu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Gavin Shan <shangw@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: ppc <linuxppc-dev@lists.ozlabs.org>