Add (as an option) slower SD transfer mode

Marlin/cardreader.cpp

@@ -146,7 +146,11 @@ void CardReader::initsd()
   cardOK = false;
   if(root.isOpen())
     root.close();
+#ifdef SDSLOW
+  if (!card.init(SPI_HALF_SPEED,SDSS))
+#else
   if (!card.init(SPI_FULL_SPEED,SDSS))
+#endif
   {
     //if (!card.init(SPI_HALF_SPEED,SDSS))
     SERIAL_ECHO_START;