Bluetooth: Host: Avoid registering callback twice

Callbacks can only be registered once. Otherwise the slist
will become circular.

In this commit we have choosen to ignore the second registration
call if the callback has already been registed. The alternative
is to trigger an assertion. That doesn't work if the assertions
are turned off.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>