Bluetooth: controller: Implement Read RSSI command

When the CONFIG_BT_CTLR_CONN_RSSI option is set, the connection RSSI is
available in the controller, and can be reported to the Host via the
Read RSSI command. Implement the command, which is required for
qualification.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>