zio: remove cset operation init() and exit()

At the moment, no known driver use these operations so we can "safely"
remove them from the framework.
We decide to remove init() and exit() because, in the current
implementation, they introduce more problems than they solve.

If you used these functions, we suggest you to move your init()
functions in the probe() function and your exit() functions before
deregister the device.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>