Bluetooth: controller: Return FICR as a static address IR

Return the IR defined in FICR as the Identity Root for the static
address through the read static addresses command instead of providing
it through the Read Key Hierarchy Root command.
This is following the recommendations in the Zephyr HCI extension
document in doc/reference/bluetooth/hci.txt

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