x86: reboot: remove inb_p usage

We are driving a motherboard port so use a 2uS explicit delay at this
point.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>