irq: change ->set_affinity() to return status, fix

This build failure:

 arch/powerpc/sysdev/mpic.c:810: error: conflicting types for 'mpic_set_affinity'
 arch/powerpc/sysdev/mpic.h:39: error: previous declaration of 'mpic_set_affinity' was here
 make[2]: *** [arch/powerpc/sysdev/mpic.o] Error 1
 make[2]: *** Waiting for unfinished jobs....

Triggers because the function prototype was not updated when the
function call signature got changed by:

   d5dedd45: irq: change ->set_affinity() to return status

[ Impact: build fix on powerpc ]

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: linux-arch@vger.kernel.org
LKML-Reference: <49F654E9.4070809@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>