Bluetooth: hci_ldisc: Provide a 'default' switch case

When both CONFIG_BT_HCIUART_INTEL and CONFIG_BT_HCIUART_BCM are not
selected, sparse complains like this:

drivers/bluetooth/hci_ldisc.c:437:9: warning: switch with no cases

Fix the sparse warning by proving a default switch case.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>