hwmon: (fam15h_power) Update running_avg_capture bit field to 28

On Carrizo and later platforms, running_avg_capture bit field is
extended to 4:31 (28 bits) from 4:25.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>