target-s390x: Fix wrong comparison in interrupt handling

gcc with -Wextra complains about an ordered pointer comparison:

target-s390x/helper.c:660:27: warning:
 ordered comparison of pointer with integer zero [-Wextra]

Obviously the index was missing in the code.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

target-s390x/helper.c

@@ -657,7 +657,7 @@ static void do_io_interrupt(CPUS390XState *env)
         cpu_unmap_lowcore(lowcore);
 
         env->io_index[isc]--;
-        if (env->io_index >= 0) {
+        if (env->io_index[isc] >= 0) {
             disable = 0;
         }
         break;