iio: dpot-dac: fix code comment in dpot_dac_read_raw()

After the replacement of the /* fall through */ comment with the
fallthrough pseudo-keyword macro, the natural reading of a code
comment was broken.

Fix the natural reading of such a comment and make it intelligible.

Reported-by: Peter Rosin <peda@axentia.se>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>