PM: Prevent dpm_prepare() from returning errors unnecessarily

Currently dpm_prepare() returns error code if it finds that a device
being suspended has a pending runtime resume request.  However, it
should not do that if the checking for wakeup events is not enabled.
On the other hand, if the checking for wakeup events is enabled, it
can return error when a wakeup event is detected, regardless of its
source.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>