rtl8192du: Fix cfg80211 warhing

Since the CFG80211 option was turned on, the following warning was issued:

WARNING: CPU: 1 PID: 7599 at net/wireless/sme.c:847 __cfg80211_disconnected+0x3a8/0x3d0 [cfg80211]()

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>