PM: hibernate: Combine return paths in power_down()

PM: hibernate: Combine return paths in power_down()

To avoid code duplication and improve clarity, combine the code
paths in power_down() leading to a return from that function.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/3571055.QJadu78ljV@rafael.j.wysocki
[ rjw: Changed the new label name to "exit" ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>