isdn: Constify some function parameters

The coming initify gcc plugin expects const pointer types, and caught
some __printf arguments that weren't const yet. This fixes those.

Signed-off-by: Emese Revfy <re.emese@gmail.com>
[kees: expanded commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>