iio:temp:mlx90614 trivial drop of unnecessary ret return from write_raw.

This is mostly part of an effort to clean out our current warnings
and make the autobuilder build reports more useful.

Still a worthwhile if trivial cleanup!

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Vianney le Clément de Saint-Marcq <vianney.leclement@essensium.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>