Bluetooth: Add a flag to indicate that SMP is going on

Add HCI_CONN_LE_SMP_PEND flag to indicate that SMP is pending
for that connection. This allows to have information that an SMP
procedure is going on for that connection.

We use the HCI_CONN_ENCRYPT_PEND to indicate that encryption
(HCI_LE_Start_Encryption) is pending for that connection.

While a SMP procedure is going on we hold an reference to the
connection, to avoid disconnections.

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