iio: hmc5843: Fix comment style warnings

Fix comment style warnings in order to comply with Coding Style
standard provided by Documentation/CodingStyle.

This patch solves following issue found with checkpatch:
WARNING: Block comments use a trailing */ on a separate line

Signed-off-by: Cristina Moraru <cristina.moraru09@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>