hwmon: (nct6683,nct6775) constify sensor_template_group structures

The sensor_template_group structures are never modified, so declare them as
const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>