powerpc/kernel: Add missing of_node_put() in legacy_serial.c

In find_legacy_serial_ports(), of_find_node_by_path() will return a node
pointer with refcount incremented. The reference should be dropped with
of_node_put() when it is not used anymore.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220619070811.4067215-1-windhl@126.com