tools: fix mavplayback.py

[tajisoft]

I got this error using mavplayback.py.

Traceback (most recent call last):
  File "/home/parallels/.local/bin/mavplayback.py", line 256, in <module>
    app=App(filename)
  File "/home/parallels/.local/bin/mavplayback.py", line 104, in __init__
    self.next_message()
  File "/home/parallels/.local/bin/mavplayback.py", line 199, in next_message
    pos = float(self.mlog.f.tell()) / self.filesize
AttributeError: 'DFReader_binary' object has no attribute 'f'