s390/ccwgroup: use BUS_NOTIFY_UNBOUND_DRIVER to trigger ungrouping

ccwgroup_notifier() currently listens for BUS_NOTIFY_UNBIND_DRIVER
events, and triggers an ungrouping for the affected device.

Looking at __device_release_driver(), we can wait for a little longer
until the driver has been fully unbound and eg. bus->remove() has been
called. So listen for BUS_NOTIFY_UNBOUND_DRIVER instead. Due to locking
the current code should work just fine, but this clarifies our intent.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>