powerpc/eeh: Fix use of set_current_state() in eeh event handling set_current_state() wart

That set_current_state() won't work very well: the subsequent mutex_lock()
might flip the task back into TASK_RUNNING.

Attempt to put it somewhere where it might have been meant to be, and
attempt to describe why it might have been added.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>