hackrf_sweep: exit early from rx_callback if do_exit set

Sometimes, if a small frequency interval is scanned, the callback is
triggered even though we already have the number of sweeps we want, and
sweep_count gets increased, showing the wrong "Total sweeps".

host/hackrf-tools/src/hackrf_sweep.c

@@ -220,6 +220,9 @@ int rx_callback(hackrf_transfer* transfer) {
 		return -1;
 	}
 
+	if(do_exit) {
+		return 0;
+	}
 	gettimeofday(&usb_transfer_time, NULL);
 	byte_count += transfer->valid_length;
 	buf = (int8_t*) transfer->buffer;