Bluetooth: hci_conn: Fix modifying handle while aborting

This introduces hci_conn_set_handle which takes care of verifying the
conditions where the hci_conn handle can be modified, including when
hci_conn_abort has been called and also checks that the handles is
valid as well.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>