hwmon: (max6639) Fix PPR register initialization to set both channels

Initialize PPR register for both channels, and set correct PPR register bits.
Also remove unnecessary variable initializations.

Signed-off-by: Chris D Schimp <silverchris@gmail.com>
[guenter.roeck@ericsson.com: Merged two patches into one]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: stable@vger.kernel.org # 3.0+
Acked-by: Roland Stigge <stigge@antcom.de>