[S390] fix clock comparator save area usage

The lowcore clock comparator save area on 64 bit machines is defined to
contain only the seven most significant bits of the register.
That's also why it starts at an uneven address (0x1331).
The current code however writes eight bytes to the address and
therefore overwrites the first byte of the access register save area.
Fix this and write only seven bytes to the save area.

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