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>

arch/powerpc/kernel/prom.c

@@ -291,11 +291,11 @@ static inline void identical_pvr_fixup(unsigned long node)
 
 static void __init check_cpu_feature_properties(unsigned long node)
 {
-	unsigned long i;
+	int i;
 	struct feature_property *fp = feature_properties;
 	const __be32 *prop;
 
-	for (i = 0; i < ARRAY_SIZE(feature_properties); ++i, ++fp) {
+	for (i = 0; i < (int)ARRAY_SIZE(feature_properties); ++i, ++fp) {
 		prop = of_get_flat_dt_prop(node, fp->name, NULL);
 		if (prop && be32_to_cpup(prop) >= fp->min_value) {
 			cur_cpu_spec->cpu_features |= fp->cpu_feature;