calibration: fix Python-to-caldac cast in CalibrationSetting._caldacs_set.

calibration: fix Python-to-caldac cast in CalibrationSetting._caldacs_set.

Pointed out by Éric Piel using Cython 0.15.1:

  Error compiling Cython file:
  ------------------------------------------------------------
  ...
              raise MemoryError()
          self.setting.num_caldacs = length
          for i,x in enumerate(value):
              if i >= length:
                  raise ValueError((i, length))
              self.setting.caldacs[i] = x
                                        ^
  ------------------------------------------------------------

  pycomedi/calibration.pyx:505:39: Cannot convert Python object to
  'comedi_caldac_t'