powerpc/prom: Remove warning on array size when empty

When neither CONFIG_ALTIVEC, nor CONFIG_VSX or CONFIG_PPC64 is
defined, the array feature_properties is defined as an empty array,
which in turn triggers the following warning (treated as error on
W=1):

  arch/powerpc/kernel/prom.c: In function ‘check_cpu_feature_properties’:
  arch/powerpc/kernel/prom.c:298:16: error: comparison of unsigned expression < 0 is always false
    for (i = 0; i < ARRAY_SIZE(feature_properties); ++i, ++fp) {
                  ^

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>