powerpc/sysfs: Replace sizeof(arr)/sizeof(arr[0]) with ARRAY_SIZE

The ARRAY_SIZE macro is more compact and more formal in linux source.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210624063632.25632-1-wangborong@cdjrlc.com