mac80211: Fix incorrect num_sta_ps decrement in __sta_info_destroy

When WLAN_STA_PS_DRIVER is set by ieee80211_sta_block_awake the
num_sta_ps counter is not incremented. Hence, we shouldn't decrement
it in __sta_info_destroy if only WLAN_STA_PS_DRIVER is set. This
could result in an incorrect num_sta_ps counter leading to strange side
effects with associated powersaving clients.

Fix this by only decrementing num_sta_ps when WLAN_STA_PS_STA was set
before.

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