Bluetooth: hci_serdev: Introduce hci_uart_unregister_device()

Several drivers have the same (and incorrect) code in their
_remove() handler.

Coalesce this into a shared function.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/drivers/bluetooth/hci_uart.h b/drivers/bluetooth/hci_uart.h
index c6e9e1c..d9cd95d 100644
--- a/drivers/bluetooth/hci_uart.h
+++ b/drivers/bluetooth/hci_uart.h
@@ -112,6 +112,7 @@ struct hci_uart {
 int hci_uart_register_proto(const struct hci_uart_proto *p);
 int hci_uart_unregister_proto(const struct hci_uart_proto *p);
 int hci_uart_register_device(struct hci_uart *hu, const struct hci_uart_proto *p);
+void hci_uart_unregister_device(struct hci_uart *hu);
 
 int hci_uart_tx_wakeup(struct hci_uart *hu);
 int hci_uart_init_ready(struct hci_uart *hu);