Previously a value of 0 was used for the RSSI to signal that this value is not present in `gnrc_netif_hdr_t`. However, an RSSI of 0 dBm is legal and even very plausible data. This commit defines `GNRC_NETIF_HDR_NO_RSSI` as `INT16_MIN`, which is below the noise floor in the vacuum of outer space and hence impossible to receive. For consistency, also GNRC_NETIF_HDR_NO_LQI is defined.