s390/sclp: fix possible control register corruption

sclp_sync_wait() disables all external interrupt classes except for
the service signal subclass.

The static mask used for that however is wrong. It clears a couple of
bits which shouldn't be cleared and on the other hand potentially does
not clear bits which should be cleared.

Fix this by using the same generic mask like we do it in our delay
implementation.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>