SDR: Fix level range of IQ date; must not exceed range of -1 .. 1

LimeSDR mini (and maybe others) do not want IQ data to become a value below
-1 or above 1. This happens slightly when applying the IIR filter after
upsampling. To prevent this, we scale down the IQ level a little bit.

To test the problem:
- set the scale level to 1.0, so it does not take effect
- use LimeSDR mini
- use upsampling (sample rate differs SDR sample rate)
- run loopback mode 2 (-l 2)
- IIR filter must be initialized with 2 or more iterations to take effect
- check linear IQ graph; it will then plot dots inside the circle

src/libsdr/sdr.c

@@ -51,6 +51,9 @@ enum paging_signal;
 /* usable bandwidth of IQ rate, because no filter is perfect */
 #define USABLE_BANDWIDTH	0.75
 
+/* limit the IQ level to prevent IIR filter from exceeding range of -1 .. 1 */
+#define LIMIT_IQ_LEVEL		0.95
+
 int sdr_rx_overflow = 0;
 
 typedef struct sdr_thread {
@@ -446,8 +449,8 @@ static void *sdr_write_child(void *arg)
 			out = sdr->thread_write.out;
 			for (s = 0, ss = 0; s < num; s++) {
 				for (o = 0; o < sdr->oversample; o++) {
-					sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out];
-					sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1];
+					sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out] * LIMIT_IQ_LEVEL;
+					sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1] * LIMIT_IQ_LEVEL;
 				}
 				out = (out + 2) % sdr->thread_write.buffer_size;
 			}