dasd: fix sysfs cleanup in dasd_generic_remove

When the DASD devices are detached from the driver, then the
dasd_generic_remove function is called. One of the things this
function should do is to remove the DASD specific sysfs attributes,
but this is not done in all cases. This is likely to cause an oops
when at a later point sysfs stumbles over the stale pointers. In
particular this happens when when the modules are unloaded and loaded
again.

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