powerpc/83xx: Add missing of_node_put() after of_device_is_available()

Add an of_node_put() when a tested device node is not available.

Fixes: c026c987 ("powerpc/83xx: Do not configure or probe disabled FSL DR USB controllers")
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>