Bluetooth: btusb: mediatek: Fix double free of skb in coredump

hci_devcd_append() would free the skb on error so the caller don't
have to free it again otherwise it would cause the double free of skb.

Fixes: 0b701513

 ("Bluetooth: btusb: mediatek: add MediaTek devcoredump support")
Reported-by : Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>