hwmon: (adt7475) Implement pwm_use_point2_pwm_at_crit

Implement the non-standard pwm_use_point2_pwm_at_crit sysfs attribute
as the adt7473 driver did.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: "Mark M. Hoffman" <mhoffman@lightlink.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>