Bluetooth: Change signature of smp_conn_security()

To make it clear that it may be called from contexts that may not have
any knowledge of L2CAP, we change the connection parameter, to receive
a hci_conn.

This also makes it clear that it is checking the security of the link.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>