[PATCH] s390: dasd set online failure

dasd driver changes:
 - The feature check in dasd_generic_online returns an error if
   the devmap entry for the device is not yet available. Check
   for the feature after the device has been created.
 - Do symmetric registration/deregistration of cdev->handler.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>