mac80211: disable power save if an infra AP vif exists

PS should not be enabled if an infra AP vif exists in
the interface list. So while recalculating PS,
AP vif type should be taken into account.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>