rtlwifi: usb: add missing freeing of skbuff

commit 36ef0b473fbf43d5db23eea4616cc1d18cec245f upstream.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>