rtlwifi: rtl8192cu: Fix unitialized struct

Driver rtl8192cu assigns a new struct rtl_tcb_desc object, but fails to
clear it.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@kernel.org>  [2.6.39+]
Signed-off-by: John W. Linville <linville@tuxdriver.com>