Fix bugs in OSTC4 support.

Fix two bugs in the support for OSTC4:
- change block size for writes back to 64 bytes to prevent buffer
  overflow on the dive computer side;
- added 'unsupported' response for the dump command on OSTC4.

Signed-off-by: Michael Keller <github@ike.ch>

src/hw_ostc3.c

@@ -240,7 +240,7 @@ hw_ostc3_write (hw_ostc3_device_t *device, dc_event_progress_t *progress, const
 	size_t nbytes = 0;
 	while (nbytes < size) {
 		// Set the maximum packet size.
-		size_t length = 1024;
+		size_t length = 64;
 
 		// Limit the packet size to the total size.
 		if (nbytes + length > size)
@@ -1719,6 +1719,10 @@ hw_ostc3_device_dump (dc_device_t *abstract, dc_buffer_t *buffer)
 		return rc;
 	}
 
+	if (device->hardware == OSTC4) {
+		return DC_STATUS_UNSUPPORTED;
+	}
+
 	// Emit a device info event.
 	dc_event_devinfo_t devinfo;
 	devinfo.firmware = device->firmware;