fix getRangingResult to properly account for signed numbers

I got numbers like this as I approached the slave (raw values before converting to meters):
```
Ranged: 8
Ranged: 6
Ranged: 3
Ranged: 3
Ranged: 5
Ranged: 5
Ranged: 5
Ranged: 6
Ranged: 1
Ranged: 1
Ranged: 800000
Ranged: 800003
Ranged: 800003
```

This is because the ToF becomes smaller than the correction factor resulting in a negative number.

This patch performs Sign Extension from 24bits to 32bits.
This result in returning a negative meter value which makes more sense than an impossibly big one.

src/modules/SX128x/SX1280.cpp

@@ -176,7 +176,8 @@ float SX1280::getRangingResult() {
   RADIOLIB_ASSERT(state);
 
   // calculate the real result
-  uint32_t raw = ((uint32_t)data[0] << 16) | ((uint32_t)data[1] << 8) | data[2];
+  uint32_t uraw = ((uint32_t)data[0] << 16) | ((uint32_t)data[1] << 8) | data[2];
+  int32_t raw = (uraw & ((1<<23) - 1)) | (uraw >> 23 << 31);
   return((float)raw * 150.0 / (4.096 * this->bandwidthKhz));
 }