PM / Runtime: Ensure timer_expires is nonzero in pm_schedule_suspend()

The runtime PM core code assumes that dev->power.timer_expires is
nonzero when the timer is scheduled, but it may become zero
incidentally in pm_schedule_suspend().  Prevent this from happening
by bumping dev->power.timer_expires up to 1 if it's 0 before calling
mod_timer().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Alan Stern <stern@rowland.harvard.edu>