PM / hibernate: no kernel_power_off when pm_power_off NULL

Reboot logic in kernel/reboot will avoid calling kernel_power_off
when pm_power_off is null, and instead uses kernel_halt.  Change
hibernate's power_down to follow the behavior in the reboot call.

Calling the notifier twice (once for SYS_POWER_OFF and again for
SYS_HALT) causes a panic during hibernation on Kirkwood
Openblocks A6 board.

Signed-off-by: Sebastian Capella <sebastian.capella@linaro.org>
Reported-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>