Bluetooth: btusb: Fix error return code in btusb_mtk_setup_firmware()

Fix to return error code -EINVAL from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: a1c49c43 ("Bluetooth: btusb: Add protocol support for MediaTek MT7668U USB devices")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>