tools/mpremote: Fix UnboundLocalError in Transport.fs_writefile().

The variable `written` was being used before it was defined in the
`fs_writefile()` method of the Transport class.  This was causing an
`UnboundLocalError` to be raised when the `progress_callback` was not
provided.

Fixes issue #16084.

Signed-off-by: Glenn Moloney <glenn.moloney@gmail.com>

tools/mpremote/mpremote/transport.py

@@ -151,9 +151,9 @@ class Transport:
             while data:
                 chunk = data[:chunk_size]
                 self.exec("w(" + repr(chunk) + ")")
-                written += len(chunk)
                 data = data[len(chunk) :]
                 if progress_callback:
+                    written += len(chunk)
                     progress_callback(written, src_size)
             self.exec("f.close()")
         except TransportExecError as e: