iwlwifi: properly handle device_set_wakeup_enable

This is intended to be set whenever wakeup will be
needed upon suspend, not only when suspending, so
use the new callbacks to set it then.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>