powerpc/pseries: Fix node leak in update_lmb_associativity_index()

In update_lmb_associativity_index() we lookup dr_node using
of_find_node_by_path() which takes a reference for us. In the
non-error case we forget to drop the reference. Note that
find_aa_index() does modify properties of the node, but doesn't need
an extra reference held once it's returned.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>