[Bluetooth] Retrieve L2CAP features mask on connection setup

The Bluetooth 1.2 specification introduced a specific features mask
value to interoperate with newer versions of the specification. So far
this piece of information was never needed, but future extensions will
rely on it. This patch adds a generic way to retrieve this information
only once per connection setup.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>