Fix bugs in ps_convert function in which the forumlas were using incorrect values.

The forumulas have been updates to match Honeywell i2c technical specifications:
http://sensing.honeywell.com/index.php?ci_id=45841

Previously they were coded using the values in the examples of how to use formulas, and not the actual formula provided. This fixes both the temperature and pressure formulas, and in doing so fixes support for Gauge (Gage) sensors.