Bluetooth: hci_uart: Cancel init work before unregistering

[ Upstream commit 3b799254cf6f481460719023d7a18f46651e5e7f ]

If hci_uart_tty_close() or hci_uart_unregister_device() is called while
hu->init_ready is scheduled, hci_register_dev() could be called after
the hci_uart is torn down. Avoid this by ensuring the work is complete
or canceled before checking the HCI_UART_REGISTERED flag.

Fixes: 9f2aee84 ("Bluetooth: Add delayed init sequence support for UART controllers")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>