rtlwifi: Fix the usage of the wrong variable in usb.c

In routine _rtl_rx_pre_process(), skb_dequeue() is called to get an skb;
however, the wrong variable name is used in subsequent calls.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>