xtensa: Enable irqs after cpu is set online

there is a small possibility that wake_up of softirq thread
happens between local_irq_enable() and set_cpu_online(). In
this case affinity of the thread changes to fallback affinity
(i.e. CPU0). This may be a source of problems.

The patch kills that possibility.

Signed-off-by: Kirill Tkhai <tkhai@yandex.ru>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>