s390/smp: use physical address for SIGP_SET_PREFIX command

Signal processor SIGP_SET_PREFIX command expects physical
address of the lowcore to be installed, but instead the
virtual address is provided.

Note: this does not fix a bug currently, since virtual and
physical addresses are identical.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>