iio: mma8452: avoid switching to active because of config change

The devices' config registers can only be changed in standby mode.
Up until now the driver just held the device *always* active, so for
changing a config it was *always* necessary to switch to standby.

For upcoming support for runtime pm, the device can as well be in standby
mode. Instead of putting runtime pm functions in there, just keep the
device in standby if it already is. This section is protected by a lock
after all.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>