Bluetooth: mediatek: fix up an error path to restore bdev->tx_state

Restore bdev->tx_state with clearing bit BTMTKUART_TX_WAIT_VND_EVT
when there is an error on waiting for the corresponding event.

Fixes: 7237c4c9 ("Bluetooth: mediatek: Add protocol support for MediaTek serial devices")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>