mac80211: Cancel the power save timer in ieee80211_stop.

Since the station info is flushed before calling set_disassoc
in ieee80211_stop, the power save timer is never cancelled
when the driver is unloaded. Hence the timer cancellation has
to be done in ieee80211_stop itself.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>