iio: accel: kxsd9: split out a common remove() function

This makes it possible to later split the transport mechanism
using a generic probe() and a generic remove().

Use dev_set_drvdata() and dev_get_drvdata() as a paired
accessor to operate on the abstract struct device * regardless
of the transport mechanism in use.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>