[S390] memory detection.

The wrong base register is used to read a value from the sclp data
structure. The value is used to calculate the memory size.
Use correct register %r4.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>