iio: accel: mma8452: define unsigned return values where appropriate

smatch warned:
	sval_binop_signed: invalid divide LLONG_MIN/-1

and this fixes it. It's actually good to have, in order to avoid accidental
checking for negative return values here.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>