There's no hot-key because the polarity is based on the waveform, not the LFO modulation range.
If you open Massive and click on any one of the LFOs, there are windows rightly to the right of the waveform display. Above them, it says "Curve Select". There are two dropdown menus below it - click on the top one and select one of those waveforms I previously mentioned! Now the modulation will only affect the green side of the modulation depth, instead of both!