rsi: add fix for crash during assertions

CVE-2018-21008

Observed crash in some scenarios when assertion has occurred,
this is because hw structure is freed and is tried to get
accessed in some functions where null check is already
present. So, avoided the crash by making the hw to NULL after
freeing.
Signed-off-by: Sanjay Konduri <sanjay.konduri@redpinesignals.com>
Signed-off-by: Sushant Kumar Mishra <sushant.mishra@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
(backported from commit abd39c6d)
[ Connor Kuehl: offset adjustment ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/wireless/rsi/rsi_91x_mac80211.c

@@ -199,6 +199,7 @@ void rsi_mac80211_detach(struct rsi_hw *adapter)
 		ieee80211_stop_queues(hw);
 		ieee80211_unregister_hw(hw);
 		ieee80211_free_hw(hw);
+		adapter->hw = NULL;
 	}
 
 	rsi_remove_dbgfs(adapter);