iio: adc: ad7298: Demote obvious misuse of kerneldoc to standard comment blocks

No attempt has been made to document either of the demoted functions here.

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

 drivers/iio/adc/ad7298.c:106: warning: Function parameter or member 'indio_dev' not described in 'ad7298_update_scan_mode'
 drivers/iio/adc/ad7298.c:106: warning: Function parameter or member 'active_scan_mask' not described in 'ad7298_update_scan_mode'
 drivers/iio/adc/ad7298.c:154: warning: Function parameter or member 'irq' not described in 'ad7298_trigger_handler'
 drivers/iio/adc/ad7298.c:154: warning: Function parameter or member 'p' not described in 'ad7298_trigger_handler'

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>