sel: Fix the deasserted thresholds inequality

The choice of > or < when printing the threshold inequality currently
only depends on whether it is an upper or lower threshold. This is
correct when the event is asserted, but when the event is deasserted the
inequality is reversed and therefore the current code can give confusing
results, e.g.:

```
Temperature ps1_inlet_temp |
Upper Non-critical going high |
Deasserted |
Reading 43.94 > Threshold 44.88 degrees C
```

This commit fixes that.

See ipmitool/ipmitool#349

Signed-off-by: Andy Clegg <andyc@graphcore.ai>