hwmon: (fam15h_power) Correct sign extension of running_avg_capture

Wrong bit was used for sign extension which caused wrong end results.
Thanks to Andre for spotting this bug.

Reported-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@vger.kernel.org