The RSSI values reported by LoRa transceiver can be less than -127.
Therefore, `int8_t` is not enough. This commit defines the RSSI of
`netdev_lora_rx_info` as `int16_t` and adapt the drivers accordingly
(sx126x, sx127x).
(cherry picked from commit 9955a35c639ca9441ad42d9b2838205bc30ed587)