Bluetooth: btmtkuart: fix error handling in mtk_hci_wmt_sync()

This code has an uninitialized variable warning:

    drivers/bluetooth/btmtkuart.c:184 mtk_hci_wmt_sync()
    error: uninitialized symbol 'wc'.

But it also has error paths which have memory leaks.

Fixes: 8f550f55b155 ("Bluetooth: btmtkuart: rely on BT_MTK module")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>