iio: magnetometer: ak8974: Add description for ak8974's 'scan' attribute

Fixes the following W=1 kernel build warning(s):

 drivers/iio/magnetometer/ak8974.c:200: warning: Function parameter or member 'scan' not described in 'ak8974'

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Samu Onkalo <samu.p.onkalo@nokia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>