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_bcsp.c b/drivers/bluetooth/hci_bcsp.c
index 50ef2e6..7d30e05 100644
--- a/drivers/bluetooth/hci_bcsp.c
+++ b/drivers/bluetooth/hci_bcsp.c
@@ -735,7 +735,7 @@
 	return 0;
 }
 
-static struct hci_uart_proto bcsp = {
+static const struct hci_uart_proto bcsp = {
 	.id		= HCI_UART_BCSP,
 	.open		= bcsp_open,
 	.close		= bcsp_close,