[S390] cio: Add css_driver_{register,unregister}.

Add wrapper functions for driver_register and driver_unregister so
that css drivers don't need to muck with struct device_driver
directly.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>