iio: adc: ad799x: Demote seemingly unintentional kerneldoc header

iio: adc: ad799x: Demote seemingly unintentional kerneldoc header

This is the only use of function related kerneldoc in the sourcefile
and no descriptions are provided.

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

 drivers/iio/adc/ad799x.c:192: warning: Function parameter or member 'irq' not described in 'ad799x_trigger_handler'
 drivers/iio/adc/ad799x.c:192: warning: Function parameter or member 'p' not described in 'ad799x_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>