sched: fix warning on ia64

Andrew Morton reported this warning on ia64:

  kernel/sched.c: In function `sd_init_NODE':
  kernel/sched.c:7449: warning: comparison of distinct pointer types lacks a cast

Using the untyped min() function produces such warnings.
Fix: type the constant 32 as unsigned int to match typeof(num_online_cpus).

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>