powerpc/powermac/udbg_scc: Add missing of_node_put()s in udbg_scc_init()

During the iteration of for_each_child_of_node(), we need to call
of_node_put() for the old references stored in to 'ch_def' and 'ch_a'
as their refcounters have been increased in last iteration.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220716074344.540049-1-windhl@126.com