net/iucv: Convert to hotplug state machine

Install the callbacks via the state machine and let the core invoke the
callbacks on the already online CPUs. The smp function calls in the
online/downprep callbacks are not required as the callback is guaranteed to
be invoked on the upcoming/outgoing cpu.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-s390@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: Ursula Braun <ubraun@linux.vnet.ibm.com>
Cc: rt@linuxtronix.de
Link: http://lkml.kernel.org/r/20161117183541.8588-13-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>