iio:dac:m62332: drop unrequired variable

A return variable is not required in _write_raw(), and dropping it reduces
complexity, as well.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>