Prevent TX exception in QPSK mode

QPSK-modulated packets require more samples, which must be available in the TX
buffer. Therefore we increase the required free space.

impl/src/main.c

@@ -248,7 +248,7 @@ int main(int argc, char **argv)
 
 				fprintf(stderr, "TX buffer free: %zu\n", buffer_free_space_samples);
 
-				if(buffer_free_space_samples < 100000) { // sample count for 50 ms at 2 MHz
+				if(buffer_free_space_samples < 400000) { // sample count for 200 ms at 2 MHz
 					// try again after a short delay
 					fsleep(10e-3);
 					continue;