Bluetooth: Fix encryption key size for peripheral role

This patch fixes the connection encryption key size information when
the host is playing the peripheral role. We should set conn->enc_key_
size in hci_le_ltk_request_evt, otherwise it is left uninitialized.

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>