samples: direction_finding_connectionless_tx: Use identity address

Use Identiy address in direction_finding_connectionless_tx sample.
Without this option the advertiser is advertising using an NRPA since
it is a non-connectable advertiser.
The function bt_le_ext_adv_oob_get_local cannot be used to get the NRPA
address of the non-connectable advertiser when privacy is disabled.

Check the return address of bt_le_ext_adv_oob_get_local.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>