mac80211: Move num_sta_ps counter decrement after synchronize_rcu

Unted the assumption that the sta struct is still accessible before the
synchronize_rcu call we should move the num_sta_ps counter decrement
after synchronize_rcu to avoid incorrect decrements if num_sta_ps.

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