Bluetooth: hci_qca: Make structure qca_proto constant

Static structure qca_proto, of type hci_uart_proto, is used four times:
as the last argument in function hci_uart_register_device(), and as the
only argument to functions hci_uart_register_proto() and
hci_uart_unregister_proto(). In all three of these functions, the
parameter corresponding to qca_proto is declared as constant. Therefore,
make qca_proto itself constant as well in order to protect it from
unintended modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>