iio: adc: rockchip_saradc: Demote seemingly unintentional kerneldoc header

iio: adc: rockchip_saradc: Demote seemingly unintentional kerneldoc header

This is the only use of kerneldoc in the source file and no
descriptions are provided.

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

 drivers/iio/adc/rockchip_saradc.c:190: warning: Function parameter or member 'reset' not described in 'rockchip_saradc_reset_controller'

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: linux-rockchip@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>