iio: magn: ak8975: make sure to power down at remove()

The code was not powering the magnetometer down properly at
remove(): just cutting the regulators without first setting the
device in power off mode. Fix this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>