PM / Driver core: leave runtime PM enabled during system shutdown

Disabling all runtime PM during system shutdown turns out not to be a
good idea, because some devices may need to be woken up from a
low-power state at that time.

The whole point of disabling runtime PM for system shutdown was to
prevent untimely runtime-suspend method calls.  This patch (as1504)
accomplishes the same result by incrementing the usage count for each
device and waiting for ongoing runtime-PM callbacks to finish.  This
is what we already do during system suspend and hibernation, which
makes sense since the shutdown method is pretty much a legacy analog
of the pm->poweroff method.

This fixes a recent regression on some OMAP systems introduced by
commit af8db150 (PM / driver core:
disable device's runtime PM during shutdown).

Reported-and-tested-by: NeilBrown <neilb@suse.de>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>