[POWERPC] Remove warning in arch/powerpc/kernel/sysfs.c

Fixes:

arch/powerpc/kernel/sysfs.c: In function 'cpu_add_sysdev_attr_group':
arch/powerpc/kernel/sysfs.c:388: warning: ignoring return value of
	'sysfs_create_group', declared with attribute warn_unused_result

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