s390/mm: fix invalid __pa() usage in pfn_pXd() macros

There is little sense in applying __pa() to a physical
address, but that what pfn_pXd() macros do.

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