hwmon: (f71882fg) Use sensor_device_attribute_2

Convert f71882fg driver from SENSOR_ATTR to SENSOR_ATTR2 use, this is a
preparation patch for adding pwm support, which is broken out to make what
changes really in the pwm support patch clear.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: Mark van Doesburg <mark.vandoesburg@hetnet.nl>
Signed-off-by: Jean Delvare <khali@linux-fr.org>