can: gs_usb: fix memory leak in gs_cmd_reset()

This patch adds the missing kfree() in gs_cmd_reset() to free the
memory that is not used anymore after usb_control_msg().

Cc: linux-stable <stable@vger.kernel.org>
Cc: Maximilian Schneider <max@schneidersoft.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>