s390/head: adjust iplstart entry point

Move iplstart entry point to 0x200 again, instead of the middle of the ipl
code. This way even the comment describing the ccw program is correct
again.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>