[PATCH] powerpc: hot_add_scn_to_nid() build fix

The return statement is to prevent `warning: 'nid' might be used uninitialized
in this function'.

Cc: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>