Bluetooth: hci_serdev: allow modular drivers

For bluetooth protocol driver only supporting serdev it makes
sense to follow common practice and built them into their own
module.

Such modules need access to hci_uart_register_device and
hci_uart_tx_wakeup for using the common protocol helpers.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>