Bluetooth: hci_ldisc: Add missing clear HCI_UART_PROTO_READY

Ensure that HCI_UART_PROTO_READY is cleared before close(hu) is
called which closes the Data Link protocol layer.

Therefore, add the missing bit clear of HCI_UART_PROTO_READY to
hci_uart_init_work() so that the flag is cleared when
hci_register_dev fails.

Without the fix, the functions of the Data Link protocol layer could
potentially be accessed after that layer has been closed. This
could lead to a crash as memory would have been freed in that layer.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>