[POWERPC] Fix warning in pseries/power.c

Introduced by commit 79393fc4


("kobject: convert pseries/power.c to kobj_attr interface").

sys_create_file takes a "struct attrbute *" not a "struct
kobj_addribute *".

arch/powerpc/platforms/pseries/power.c: In function 'apo_pm_init':
arch/powerpc/platforms/pseries/power.c:78: warning: passing argument 2 of 'sysfs_create_file' from incompatible pointer type

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>