rtlwifi: usb: add missing freeing of skbuff

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>