Bluetooth: hci_ll: Use new hci_uart_unregister_device() function

Convert hci_ll to use hci_uart_unregister_device().

This simplifies the _remove() handler as well as fixes a
potential race condition on unload.

Signed-off-by: Ian Molton <ian@mnementh.co.uk>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabor.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>