Commented out ctypes.c_float_p casts

[cjeanneret]

There is no such attribute for ctypes at least on Python 2.7 (current Debian Squeeze version). More over, Python ctypes documentation doesn't speak of this attribute: http://docs.python.org/2/library/ctypes.html#fundamental-data-types

After some tests, it seems that this modification doesn't affect in a bad maner the library.

Previous behavior: Fails on main.actions.manualfocusdrive (value according to gphoto2 itself: 0)

Current behavior: main.actions.manualfocusdrive = None

Tested with a Nikon d90.