s390/mm: use BIT macro to generate SET_MEMORY bit masks

Use BIT macro to generate SET_MEMORY bit masks, which is easier to
maintain if bits get added, or removed.

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