Bluetooth: hci_ldisc: Add missing return in hci_uart_init_work()

If hci_register_dev() returns an error in hci_uart_init_work()
then the HCI_UART_REGISTERED bit gets erroneously set due to
a missing return statement. Therefore, add the missing return
statement.

The consequence of the missing return is that the HCI UART is not
registered but HCI_UART_REGISTERED is set which allows the code
to think that hu->hdev is safe to access but hu->hdev has been
freed so could lead to a crash.

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