mac80211: clear sta.drv_priv on reconfiguration

drivers might assume sta.drv_priv is clear while
the sta is added, so clear it on reconfinguration.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>