iio: mlx90614: Define magic numbers

Translates the magic constant numbers to named macros and add some
additional comments about their meaning.

The diff is made towards togreg branch as that branch seems to have the
most recent updates of mlx90614 driver (many are yet to be merged).

Signed-off-by: Crt Mori <cmo@melexis.com>
Acked-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>