Minor fix to read back the right amout of I2C data from the interface…

… mcu. (#159)

A bit inconsequential as the data read back is not being used, but
the interface firmware is meant to send back the same number of bytes
that was written to it.

source/MicroBitUSBFlashManager.cpp

@@ -162,7 +162,7 @@ int MicroBitUSBFlashManager::setConfiguration(MicroBitUSBFlashConfig config, boo
 
     // Write out each of the parameters in turn.
     transact(fname, 12);
-    transact(fsize, 2);
+    transact(fsize, 5);
     transact(fvisible, 2);
 
     if (persist)