iio: adc: twl6030-gpadc: Fix some misdocumentation and formatting issues

Kerneldoc expects attributes/parameters to be in '@*.: ' format.

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

 drivers/iio/adc/twl6030-gpadc.c:110: warning: Function parameter or member 'ideal' not described in 'twl6030_gpadc_platform_data'
 drivers/iio/adc/twl6030-gpadc.c:110: warning: Function parameter or member 'channel_to_reg' not described in 'twl6030_gpadc_platform_data'

Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Nishant Kamat <nskamat@ti.com>
Cc: Balaji T K <balajitk@ti.com>
Cc: Graeme Gregory <gg@slimlogic.co.uk>
Cc: Girish S Ghongdemath <girishsg@ti.com>
Cc: Ambresh K <ambresh@ti.com>
Cc: Oleksandr Kozaruk <oleksandr.kozaruk@ti.com>
Cc: Mikko Ylinen <mikko.k.ylinen@nokia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>