staging:iio:adc:ad7280a: Trivial comment formatting cleanup

IIO uses the
/*
 * stuff
 * more stuff
 */

multi-line style, so use that here as well.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-10-jic23@kernel.org