MavlinkLogHandler: increase MAX_BYTES_SEND to 256kb

It increases the throughput on UDP (from around 2Mb to 2.5Mb), while the
rate via USB & telemetry stay the same.

src/modules/mavlink/mavlink_log_handler.cpp

@@ -133,7 +133,7 @@ void
 MavlinkLogHandler::send(const hrt_abstime /*t*/)
 {
 	//-- An arbitrary count of max bytes in one go (one of the two below but never both)
-#define MAX_BYTES_SEND 64 * 1024
+#define MAX_BYTES_SEND 256 * 1024
 	size_t count = 0;
 
 	//-- Log Entries