powerpc/embedded6xx/ls_uart: Add missing of_node_put()

In ls_uarts_init(), add an of_node_put() to keep refcount balance.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220620065904.4071787-1-windhl@126.com