mac80211: allow using AP_LINK_PS with mac80211-generated TIM IE

In 46fa38e8

 ("mac80211: allow software PS-Poll/U-APSD with
AP_LINK_PS"), Johannes allowed to use mac80211's code for handling
stations that go to PS or send PS-Poll / uAPSD trigger frames for
devices that enable RSS.

This means that mac80211 doesn't look at frames anymore but rather
relies on a notification that will come from the device when a PS
transition occurs or when a PS-Poll / trigger frame is detected by
the device.

iwlwifi will need this capability but still needs mac80211 to take
care of the TIM IE. Today, if a driver sets AP_LINK_PS, mac80211
will not update the TIM IE. Change mac80211 to check existence of
the set_tim driver callback rather than using AP_LINK_PS to decide
if the driver handles the TIM IE internally or not.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
[reword commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>