Typo in getting mux data levels

[lukabavdaz]

To be consistent with the documentation and with setting the data levels, the positive and negative should be swapped.

SPI-rack/spirack/U2_module.py

Lines 119 to 120 in baaf5a5

	return [self.voltages[U2_module.DAC_data_pos],
	self.voltages[U2_module.DAC_data_neg]]