PM / Runtime: Handle .runtime_suspend() failure correctly

If .runtime_suspend() returns -EAGAIN or -EBUSY, the device should
still be in ACTIVE state, so it is not necessary to send an idle
notification to its parent.  If .runtime_suspend() returns other
fatal failure, it doesn't make sense to send idle notification to
its parent.

Skip parent idle notification when failure is returned from
.runtime_suspend() and update comments in rpm_suspend() to reflect
that change.

[rjw: Modified the subject and changelog slightly.]

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>