ad8232 issues

[jontrulson]

This sensor does not output a heart rate - it's meant as an EKG-like
sensor. The analog values should be plotted and the effect is like
looking at an EKG machine. An oscilloscope in 'roll mode' also
works well.

Also, removed the upm_heart_rate_u from driver get_value(). If
drivers should support this, then these enums should be moved into
UPM (non-FTI) specific headers. But in any case, it's irrelevant
with this particular driver.