[POWERPC] Remove warning in mpic.c

arch/powerpc/sysdev/mpic.c: In function 'mpic_request_ipis':
arch/powerpc/sysdev/mpic.c:1445: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>