s390/numa: let NODES_SHIFT depend on NEED_MULTIPLE_NODES

Qian Cai reported:
"""
When NUMA=n and nr_node_ids=2, in apply_wqattrs_prepare(), it has,

for_each_node(node) {
        if (wq_calc_node_cpumask(...

where it will trigger a booting warning,

WARNING: workqueue cpumask: online intersect > possible intersect

because it found 2 nodes and wq_numa_possible_cpumask[1] is an empty
cpumask.
"""

Let NODES_SHIFT depend on NEED_MULTIPLE_NODES like it is done
on other architectures in order to fix this.

Fixes: 701dc81e ("s390/mm: remove fake numa support")
Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>