hott:Fix compiler error array bound is unknown

This fixes error: variable-length array bound is unknown [-Werror=vla-larger-than=]
   with gcc 9.1

src/drivers/telemetry/hott/hott_sensors/hott_sensors.cpp

@@ -89,7 +89,7 @@ send_poll(int uart, uint8_t *buffer, size_t size)
 
 	/* A hack the reads out what was written so the next read from the receiver doesn't get it. */
 	/* TODO: Fix this!! */
-	uint8_t dummy[size];
+	uint8_t dummy[MAX_MESSAGE_BUFFER_SIZE];
 	read(uart, &dummy, size);
 
 	return OK;

src/drivers/telemetry/hott/hott_telemetry/hott_telemetry.cpp

@@ -157,7 +157,7 @@ send_data(int uart, uint8_t *buffer, size_t size)
 
 	/* A hack the reads out what was written so the next read from the receiver doesn't get it. */
 	/* TODO: Fix this!! */
-	uint8_t dummy[size];
+	uint8_t dummy[MAX_MESSAGE_BUFFER_SIZE];
 	read(uart, &dummy, size);
 
 	return PX4_OK;