iwlwifi: clean up (wowlan) suspend flow

In the WoWLAN suspend flow, instead of accessing
registers directly, ask the transport to do the
required setup at the end of suspend. If the
transport doesn't implement this, don't tell the
stack we support WoWLAN.

When the device suspends w/o WoWLAN, mac80211
will have stopped it already, which has already
called iwl_apm_stop() via stop_hw(). Thus, it
isn't necessary to call it again in pcie_suspend
and we can simply do nothing there.

This unifies the regular and WoWLAN suspend.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>