Bluetooth: controller: Enforce support for Read RSSI command in Kconfig

Enforce support for Read RSSI command if BT_CONN and BT_HCI_RAW are
selected. According to the Bluetooth specification, support for the
Read RSSI command is mandatory if the Connection State is
supported. BT_HCI_RAW indicates that the controller is an
independently qualifiable module and not part of a combined
host/controller build. This change avoids a configuration that cannot
be possibly qualified.

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>