pid: rename variables with full names
pid: rename variables with full names

Use full names and camelcase for the variables in the pid loop to
improve readability and consistency.

Change-Id: I86be69d94d3008faa497eace050d1f3b9b6a9ff4
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-pid-control,openbmc/phosphor-pid-control