[S390] dasd: fix bug in dasd initialization cleanup

The initialization of the dasd_eer code is one of the last steps of the
dasd driver initialization. When initialization fails in one of the
earlier steps, the dasd_exit function is called to clean up what has been
done so far. So the dasd_eer_exit function may be called, although the
dasd_eer_init function wasn't called before and dasd_eer_exit tries to
unregister a misc device that wasn't registered, which results in a BUG.

Make sure that dasd_eer_exit can be called without initialization. Use a
dynamically allocated struct miscdevice instead of a static one, so we
only try to unregister the device if it exists and was actually registered.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>