x86, percpu: fix minor bugs in setup_percpu.c

Recent changes in setup_percpu.c made a now meaningless DBG()
statement fail to compile and introduced a
comparison-of-different-types warning.  Fix them.

Compile failure is reported by Ingo Molnar.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ingo Molnar <mingo@elte.hu>