s390/sclp: disable FORTIFY_SOURCE for early sclp code

Michal Suchánek reported the following compile error with
FORTIFY_SOURCE enabled:

drivers/s390/char/sclp_early_core.o: In function `memcpy':
include/linux/string.h:340: undefined reference to `fortify_panic'

To fix this simply disable FORTIFY_SOURCE on the early sclp code as
well, which I forgot on the initial commit.

Fixes: 79962038

 ("s390: add support for FORTIFY_SOURCE")
Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>