Bluetooth: Refactor in hci_le_conn_complete_evt

This patch moves the hci_conn check to begining of hci_le_conn_
complete_evt in order to improve code's readability and better
error handling.

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