Bluetooth: controller: legacy: Fix conn RSSI initial value

On connection callback generated by host, application
reading the RSSI value by using HCI LE Read RSSI Command
received -127 dB as the first connection event has not
occured yet in order to measure the RSSI value.

Fix this by using the RSSI value of received advertising
PDU by the initiator, and using the RSSI value of the
received CONNECT_REQ PDU by the connectable advertiser as
the initial value at connection setup.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>