mac80211: fix tim recalculation after PS response

Handle the case where the mac80211 intermediate queues are empty and the
driver has buffered frames

Fixes: ba8c3d6f

 ("mac80211: add an intermediate software queue implementation")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>