Bluetooth: allow constant arguments for bacmp()/bacpy()

There is no reason to require the source arguments to be writeable so fix
this to allow constant source addresses.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>