Bluetooth: hci_uart: Make struct hci_uart_proto always const

The usage of struct hci_uart_proto should always be const. Change the
function headers and individual protocol drivers.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
diff --git a/drivers/bluetooth/hci_h5.c b/drivers/bluetooth/hci_h5.c
index 0f681cc1..aac0e8f 100644
--- a/drivers/bluetooth/hci_h5.c
+++ b/drivers/bluetooth/hci_h5.c
@@ -743,7 +743,7 @@
 	return 0;
 }
 
-static struct hci_uart_proto h5p = {
+static const struct hci_uart_proto h5p = {
 	.id		= HCI_UART_3WIRE,
 	.open		= h5_open,
 	.close		= h5_close,