iio: mma8452: use runtime pm instead of device specific autosleep

What is this autosleep?
-----------------------
It slows down the device after x seconds of inactivity. The thing is, we have
really achieved almost the same by runtime pm.

differnces are:

autosleep
 * uses more power during inactivity
 * the first read after inactivity slightly faster
 * complicated to understand for the user
 * no documented sysfs interface (afaik)
 * complicated to read and maintain

runtime pm
 * already merged in mma8452
 * uses less power during inactivity
 * first read after inactivity slower
 * easy to use. well documented.
 * easy to maintain and understand

The two approaches solve the same problem. runtime pm has more advantages
than autosleep and comes quite close to it's behaviour anyways. As I see it,
autosleep, even if somehow supported, would never be used anyways.

So resolve this issue by "ignoring" autosleep.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Reviewed-by: Martina Kepplinger <martina.novakovic@zoho.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>