s390/cio: cleanup cmb_enable sysfs attr

Ensure that we hold the ccwlock when accessing private data. Return errors
that occur during measurement enabling to userspace. Apply some cleanups
while at it.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>