iio: adc: ad7923: 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/ad7923.c:159: warning: Function parameter or member 'indio_dev' not described in 'ad7923_update_scan_mode'
 drivers/iio/adc/ad7923.c:159: warning: Function parameter or member 'active_scan_mask' not described in 'ad7923_update_scan_mode'
 drivers/iio/adc/ad7923.c:202: warning: Function parameter or member 'irq' not described in 'ad7923_trigger_handler'
 drivers/iio/adc/ad7923.c:202: warning: Function parameter or member 'p' not described in 'ad7923_trigger_handler'

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Patrick Vasseur <patrick.vasseur@c-s.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>