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

commit 0a06ad8e3a1cb5311b7dbafde45410aa1bce9d40 upstream.

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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>