Bluetooth: host: Fix directed advertiser for multiple local identities

Fix directed advertiser using the wrong local IRK when doing directed
advertising when the identity of the advertiser is not the default
identity.
This is only an issue for the directed advertiser because it is only
for the directed advertiser that we use the controllers local IRK
to generate the Advertisers RPA.

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