s390/sclp: fix Secure-IPL facility detection

Prevent out-of-range access if the returned SCLP SCCB response is smaller
in size than the address of the Secure-IPL flag.

Fixes: c9896acc ("s390/ipl: Provide has_secure sysfs attribute")
Cc: stable@vger.kernel.org # 5.2+
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>