mac80211: Fix incorrect num_sta_ps decrement in ap_sta_ps_end

If the driver blocked this specific STA with the help of
ieee80211_sta_block_awake we won't clear WLAN_STA_PS_STA later but
still decrement num_sta_ps. Hence, the next data frame from this
STA will trigger ap_sta_ps_end again and also decrement num_sta_ps
again leading to an incorrect num_sta_ps counter.

This can result in problems with powersaving clients not waking up
from PS because the TIM calculation might be skipped due to the
incorrect num_sta_ps counter.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>