Bluetooth: btmtkuart: fix a memleak in mtk_hci_wmt_sync

bdev->evt_skb will get freed in the normal path and one error path
of mtk_hci_wmt_sync, while the other error paths do not free it,
which may cause a memleak. This bug is suggested by a static analysis
tool, please advise.

Fixes: e0b67035 ("Bluetooth: mediatek: update the common setup between MT7622 and other devices")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>