s390/sclp: disable early sclp code as soon as the base sclp driver is active

Make sure the early sclp code does not generate any sclp requests
anymore as soon as the base sclp driver is active. Otherwise both
drivers may see unexpected requests or may miss expected interrupts.

Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>