Currently the error return path when the call to btusb_mtk_hci_wmt_sync
fails does not free fw.  Fix this by returning via the error_release_fw
label that performs the free'ing.

Addresses-Coverity: ("Resource leak")
Fixes: a1c49c43 ("Bluetooth: btusb: Add protocol support for MediaTek MT7668U USB devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>