Bluetooth: l2cap: reword unknown pdu warning

This is a scary-looking warning for users, and should only really matter to
stack developers. It will eventually show up in the wild for embedded
devices that don't get updated to the latest and greatest.

Reword it to make it clear we are rejecting it, and it doesn't impact the
stack in any way.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>