powerpc/mpic: Fix ambiguous else statement

This fixes the following warning, introduced by commit
475ca391 (mpic: Deal with bogus NIRQ
in Feature Reporting Register):

  CC      arch/powerpc/sysdev/mpic.o
arch/powerpc/sysdev/mpic.c: In function 'mpic_alloc':
arch/powerpc/sysdev/mpic.c:1146: warning: suggest explicit braces to avoid ambiguous 'else'

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>