powerpc/pseries: Fix missing of_node_put() in rng_init()

The call to of_find_compatible_node() returns a node pointer with
refcount incremented thus it must be explicitly decremented here
before returning.

Fixes: a489043f ("powerpc/pseries: Implement arch_get_random_long() based on H_RANDOM")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1530522496-14816-1-git-send-email-hofrat@osadl.org