PM / Hibernate: Fix snapshot error code path

There is an inconsistency between hibernation_platform_enter()
and hibernation_snapshot(), because the latter calls
hibernation_ops->end() after failing hibernation_ops->begin(), while
the former doesn't do that.  Make hibernation_snapshot() behave in
the same way as hibernation_platform_enter() in that respect.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Len Brown <len.brown@intel.com>