Bluetooth: Fix security level for peripheral role

While playing the peripheral role, the host gets a LE Long Term Key
Request Event from the controller when a connection is established
with a bonded device. The host then informs the LTK which should be
used for the connection. Once the link is encrypted, the host gets
an Encryption Change Event.

Therefore we should set conn->pending_sec_level instead of conn->
sec_level in hci_le_ltk_request_evt. This way, conn->sec_level is
properly updated in hci_encrypt_change_evt.

Moreover, since we have a LTK associated to the device, we have at
least BT_SECURITY_MEDIUM security level.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>