s390/iucv: fix receive buffer virtual vs physical address confusion

Fix IUCV_IPBUFLST-type buffers virtual vs physical address confusion.
This does not fix a bug since virtual and physical address spaces are
currently the same.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>