Bluetooth: Fix initial security level of LE links

As the default security level (BT_SECURITY_SDP) doesn't make sense for
LE links, initialize LE links with something that makes sense.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>