Bluetooth: Host: Refactor `bt_setup_public_id_addr`

Move the flagging of `BT_DEV_STORE_ID` to a separate if-statement.

This clarifies the meaning of the code, and paves way for wrapping the
if-statement with the call to `bt_smp_irk_get` in another conditional.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>